feat(caldav): link-existing picker + projax-tagged VTODOs for shared lists

m's ask: per-item CalDAV linking should support existing lists, not
just create-new. Athena's design update extended it: also tag VTODOs
on create so multiple projax items can SHARE one CalDAV list, with
projax doing tag-based slicing on read.

Three layers, one branch:

## 1. Link-existing picker (the original ask)

- New POST /i/{path}/caldav/link-existing handler validates the
  submitted calendar_url is in the discoverable PROPFIND set (defence
  against crafted forms pointing at arbitrary HTTP servers), then
  inserts the item_link row with display_name + color metadata
  preserved from the discovery payload.
- handleDetail + renderTasksSection pre-load
  availableCalendarsForItem(ctx, links) — calendars from
  s.CalDAV.Client.ListCalendars MINUS the ones already linked to this
  item. Errors degrade to an empty picker (non-fatal).
- tasks_section.tmpl gains a .caldav-actions block rendering the
  picker (<select> of available calendars) when AvailableCalendars
  is non-empty AND the Create-new button (when the item has no
  linked list yet). Same surface serves both the "first link" flow
  and the "+ link another" flow per athena's brief.

## 2. Tag-on-create (CATEGORIES carries projax:<path>)

- caldav package gains Categories []string on Todo + the same on
  VTodoEdit. BuildVTodoICS emits a CATEGORIES line when non-empty;
  parseVTodos parses CATEGORIES comma-list into the slice with per-
  entry unescape per RFC 5545.
- handleCalDAVTodoAction action="todo-create" passes
  `Categories: []{ProjaxCategoryFor(it.PrimaryPath())}` into
  VTodoEdit so every per-item Add submits a tagged VTODO.
- ApplyVTodoEdit intentionally ignores the Categories field —
  edit/complete/delete paths preserve existing CATEGORIES via the
  unknown-property pass-through that's been tested since Phase 5
  (TestApplyVTodoEditPreservesUnknown).

## 3. Per-item filter (managed-vs-legacy)

- detailTodos now calls caldav.AnyTodoHasProjaxTag(todos) to decide
  whether the linked list is projax-managed (any projax: tag
  anywhere) or legacy/unmanaged (zero projax: tags).
  - Managed → filter to VTODOs whose CATEGORIES include this
    item's projax:<path>. Multiple projax: tags are AND-of-OR — a
    VTODO with two projax tags appears on both items per athena's
    multi-tag contract.
  - Legacy → show every VTODO untouched. Existing pre-5j users with
    untagged lists keep seeing everything; the detail page doesn't
    suddenly hide their tasks.

## Helpers (caldav package, exported)

- ProjaxCategoryFor(primaryPath) → "projax:<path>" string
- HasProjaxTag(t) bool → any projax: prefix
- HasProjaxTagFor(t, primaryPath) bool → exact projax:<path>
- AnyTodoHasProjaxTag(todos) bool → list-level signal

## Tests

caldav unit (caldav/projax_tags_test.go):
- TestProjaxCategoryFor / TestHasProjaxTagAndFor /
  TestAnyTodoHasProjaxTag / TestBuildVTodoICSEmitsCategories /
  TestParseVTodosMultiCategory.

web integration (web/caldav_link_existing_test.go) — single fake
CalDAV server (httptest) answering PROPFIND + REPORT + PUT, then
four end-to-end probes:
- TestDetailLinkExistingCalendar — three calendars discoverable,
  picker renders, POST link-existing creates the link, second GET
  drops the linked URL from the picker.
- TestVTodoCreateAttachesProjaxCategory — Add-task POST writes a
  VTODO whose CATEGORIES contains projax:<path>.
- TestDetailFilterByProjaxCategory — one calendar shared between
  Trip A and Trip B with three tagged VTODOs; A sees A+shared,
  B sees B+shared, neither sees the other's tagged-only VTODO.
- TestDetailUntaggedListShowsAll — linked list with zero projax
  tags renders ALL VTODOs (legacy fallback).

Full web + caldav suites green. Pre-existing
db/TestBackfillTagsFromArea failure unchanged.

Net: +795 / -14.

caldav/caldav.go

@@ -55,6 +55,11 @@ type Todo struct {
 	Due          *time.Time
 	Priority     int
 	LastModified *time.Time
+	// Categories carries the RFC 5545 CATEGORIES property as a flat
+	// slice (already comma-split, trimmed). Phase 5j uses entries
+	// prefixed `projax:<primary-path>` to tag VTODOs to projax items —
+	// see HasProjaxTag + ProjaxCategoryFor in this package.
+	Categories []string
 	URL        string // absolute URL of the .ics resource on the server
 	ETag       string // server-issued ETag; pass to PutTodo/DeleteTodo as If-Match
 	Raw        string // raw VCALENDAR ICS as returned by the server, preserved for in-place edits

caldav/parse.go

@@ -54,11 +54,70 @@ func parseVTodos(ics string) []Todo {
 			if t, ok := parseICalTime(val); ok {
 				cur.LastModified = &t
 			}
+		case "CATEGORIES":
+			// CATEGORIES is comma-separated per RFC 5545. Some clients emit
+			// multiple CATEGORIES lines; we merge by appending. The unescape
+			// is per-entry because commas inside a category value MUST be
+			// escaped (`\,`), so we split on bare commas only after unescape.
+			for _, raw := range strings.Split(val, ",") {
+				t := strings.TrimSpace(unescapeText(raw))
+				if t == "" {
+					continue
+				}
+				cur.Categories = append(cur.Categories, t)
+			}
 		}
 	}
 	return out
 }
 
+// ProjaxCategoryFor returns the projax-namespaced CATEGORIES entry for
+// the given primary-path (e.g. "projax:admin.vacations.greece"). Used by
+// both the write side (tag-on-create) and the read side (per-item filter).
+func ProjaxCategoryFor(primaryPath string) string {
+	return "projax:" + primaryPath
+}
+
+// HasProjaxTag reports whether the VTODO carries any `projax:` category.
+// Used to decide whether the per-item filter kicks in: a list with at
+// least one projax: tag is "managed" by projax and the detail page only
+// shows todos matching THIS item's path; a list with zero projax: tags
+// is a legacy/unmanaged list and the detail page shows everything.
+func HasProjaxTag(t Todo) bool {
+	for _, c := range t.Categories {
+		if strings.HasPrefix(c, "projax:") {
+			return true
+		}
+	}
+	return false
+}
+
+// HasProjaxTagFor reports whether the VTODO carries the specific
+// `projax:<primaryPath>` category. A todo can carry multiple projax: tags
+// (when it belongs to multiple projax items) — any match returns true.
+func HasProjaxTagFor(t Todo, primaryPath string) bool {
+	want := ProjaxCategoryFor(primaryPath)
+	for _, c := range t.Categories {
+		if c == want {
+			return true
+		}
+	}
+	return false
+}
+
+// AnyTodoHasProjaxTag reports whether the slice contains at least one
+// projax-tagged VTODO. The detail page uses this to decide between the
+// projax-managed filter (show only matching) and the legacy unmanaged
+// path (show all).
+func AnyTodoHasProjaxTag(todos []Todo) bool {
+	for _, t := range todos {
+		if HasProjaxTag(t) {
+			return true
+		}
+	}
+	return false
+}
+
 // parseVEvents extracts every VEVENT block from a calendar-data string.
 // Mirrors parseVTodos but for read-only event listing (no writeback). DTSTART
 // with VALUE=DATE marks the event all-day; the parser inspects the raw line
@@ -296,6 +355,13 @@ type VTodoEdit struct {
 	Due       *time.Time
 	ClearDue  bool
 	Priority  *int
+	// Categories: optional CATEGORIES list. BuildVTodoICS writes them
+	// directly on a fresh VTODO. ApplyVTodoEdit intentionally ignores
+	// this field — existing categories pass through unchanged via the
+	// unknown-property preserve path, which is what every edit/complete/
+	// delete flow wants. Tag-on-create is the only write path that
+	// uses it.
+	Categories []string
 }
 
 // BuildVTodoICS serialises a fresh VTODO as a complete VCALENDAR document,
@@ -336,6 +402,15 @@ func BuildVTodoICS(uid string, e VTodoEdit) string {
 	if e.Priority != nil {
 		lines = append(lines, fmt.Sprintf("PRIORITY:%d", *e.Priority))
 	}
+	if len(e.Categories) > 0 {
+		// RFC 5545 CATEGORIES — comma-separated, single line. Escape commas
+		// inside individual entries so the round-trip survives parseVTodos.
+		escaped := make([]string, 0, len(e.Categories))
+		for _, c := range e.Categories {
+			escaped = append(escaped, escapeText(c))
+		}
+		lines = append(lines, "CATEGORIES:"+strings.Join(escaped, ","))
+	}
 	lines = append(lines, "END:VTODO", "END:VCALENDAR")
 	return joinICS(lines)
 }

caldav/projax_tags_test.go

@@ -0,0 +1,114 @@
+package caldav
+
+import (
+	"strings"
+	"testing"
+)
+
+// TestProjaxCategoryFor pins the tag string format. The format is part
+// of the projax↔CalDAV contract — `projax:<primary-path>` — and other
+// tooling (admin triage, future migration scripts) will rely on the
+// prefix. A typo here silently breaks the per-item filter.
+func TestProjaxCategoryFor(t *testing.T) {
+	got := ProjaxCategoryFor("admin.vacations.greece")
+	want := "projax:admin.vacations.greece"
+	if got != want {
+		t.Errorf("ProjaxCategoryFor = %q, want %q", got, want)
+	}
+}
+
+// TestHasProjaxTagAndFor exercises the two read-side helpers that drive
+// the per-item filter on the detail page: HasProjaxTag (any projax: tag
+// at all) and HasProjaxTagFor (matches THIS path).
+func TestHasProjaxTagAndFor(t *testing.T) {
+	tagged := Todo{Categories: []string{"home", "projax:admin.vacations.greece", "errands"}}
+	if !HasProjaxTag(tagged) {
+		t.Errorf("HasProjaxTag should fire for any projax: category")
+	}
+	if !HasProjaxTagFor(tagged, "admin.vacations.greece") {
+		t.Errorf("HasProjaxTagFor should match exact projax:<path>")
+	}
+	if HasProjaxTagFor(tagged, "admin.vacations.spain") {
+		t.Errorf("HasProjaxTagFor should NOT match a different path")
+	}
+
+	multi := Todo{Categories: []string{"projax:work.proj1", "projax:work.proj2"}}
+	if !HasProjaxTagFor(multi, "work.proj1") {
+		t.Errorf("multi-tag todo should match first projax: tag")
+	}
+	if !HasProjaxTagFor(multi, "work.proj2") {
+		t.Errorf("multi-tag todo should match second projax: tag")
+	}
+
+	untagged := Todo{Categories: []string{"home", "errands"}}
+	if HasProjaxTag(untagged) {
+		t.Errorf("HasProjaxTag should be false on a no-projax: list")
+	}
+	if HasProjaxTagFor(untagged, "anything") {
+		t.Errorf("HasProjaxTagFor must be false when no projax: tag exists")
+	}
+}
+
+// TestAnyTodoHasProjaxTag drives the list-level managed-vs-legacy
+// decision in detailTodos. Untagged lists keep their pre-5j show-all
+// behaviour; one tagged todo flips the entire list into managed mode.
+func TestAnyTodoHasProjaxTag(t *testing.T) {
+	none := []Todo{
+		{Categories: []string{"home"}},
+		{Categories: nil},
+	}
+	if AnyTodoHasProjaxTag(none) {
+		t.Errorf("untagged list should NOT be projax-managed")
+	}
+	mixed := []Todo{
+		{Categories: []string{"home"}},
+		{Categories: []string{"projax:admin.vacations.greece"}},
+	}
+	if !AnyTodoHasProjaxTag(mixed) {
+		t.Errorf("list with one projax-tagged todo should be projax-managed")
+	}
+}
+
+// TestBuildVTodoICSEmitsCategories proves the tag-on-create path. The
+// Phase 5j write side passes Categories into VTodoEdit; BuildVTodoICS
+// must render the CATEGORIES line so the server-side round-trip
+// (parseVTodos picks it back up) carries the tag through.
+func TestBuildVTodoICSEmitsCategories(t *testing.T) {
+	summary := "Buy gear"
+	ics := BuildVTodoICS("uid-tagged", VTodoEdit{
+		Summary:    &summary,
+		Categories: []string{"projax:admin.vacations.greece"},
+	})
+	if !strings.Contains(ics, "CATEGORIES:projax:admin.vacations.greece") {
+		t.Errorf("BuildVTodoICS should emit CATEGORIES line, got:\n%s", ics)
+	}
+	// Round-trip: parse it back, the Categories slice must be populated.
+	todos := parseVTodos(ics)
+	if len(todos) != 1 {
+		t.Fatalf("parseVTodos round-trip expected 1 todo, got %d", len(todos))
+	}
+	if !HasProjaxTagFor(todos[0], "admin.vacations.greece") {
+		t.Errorf("round-trip lost CATEGORIES: %#v", todos[0].Categories)
+	}
+}
+
+// TestParseVTodosMultiCategory proves the parser handles RFC 5545
+// comma-separated CATEGORIES correctly (a single CATEGORIES line with
+// multiple values, not multiple CATEGORIES lines). This is the wire
+// shape Apple Calendar + Thunderbird + Radicale all emit.
+func TestParseVTodosMultiCategory(t *testing.T) {
+	ics := "BEGIN:VCALENDAR\r\nVERSION:2.0\r\nBEGIN:VTODO\r\nUID:multi\r\nSUMMARY:Multi\r\nSTATUS:NEEDS-ACTION\r\nCATEGORIES:home,projax:admin.vacations.greece,projax:work.someproj,errands\r\nEND:VTODO\r\nEND:VCALENDAR\r\n"
+	todos := parseVTodos(ics)
+	if len(todos) != 1 {
+		t.Fatalf("expected 1 todo, got %d", len(todos))
+	}
+	want := []string{"home", "projax:admin.vacations.greece", "projax:work.someproj", "errands"}
+	if len(todos[0].Categories) != len(want) {
+		t.Fatalf("Categories = %v, want %v", todos[0].Categories, want)
+	}
+	for i, c := range todos[0].Categories {
+		if c != want[i] {
+			t.Errorf("Categories[%d] = %q, want %q", i, c, want[i])
+		}
+	}
+}

web/caldav.go

@@ -151,6 +151,93 @@ func (s *Server) handleCalDAVUnlink(w http.ResponseWriter, r *http.Request) {
 	http.Redirect(w, r, "/admin/caldav", http.StatusSeeOther)
 }
 
+// availableCalendarsForItem returns the discoverable CalDAV calendars
+// minus the ones already linked to this item — feeds the per-item
+// "Link existing list" picker on the detail page. Errors during
+// discovery (network, auth, parse) are surfaced to the caller; callers
+// downgrade to an empty list so the rest of the page still renders.
+//
+// "Already linked" is computed by the caller's `links` slice rather
+// than a fresh fetch, since handleDetail/renderTasksSection already
+// loaded the per-item caldav-list links inside detailTodos and we
+// avoid a second LinksByType round-trip.
+func (s *Server) availableCalendarsForItem(ctx context.Context, links []*store.ItemLink) ([]caldav.Calendar, error) {
+	if s.CalDAV == nil {
+		return nil, nil
+	}
+	cals, err := s.CalDAV.Client.ListCalendars(ctx)
+	if err != nil {
+		return nil, err
+	}
+	linkedURLs := map[string]struct{}{}
+	for _, l := range links {
+		linkedURLs[l.RefID] = struct{}{}
+	}
+	out := make([]caldav.Calendar, 0, len(cals))
+	for _, c := range cals {
+		if _, already := linkedURLs[c.URL]; already {
+			continue
+		}
+		out = append(out, c)
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].DisplayName < out[j].DisplayName })
+	return out, nil
+}
+
+// handleCalDAVLinkExisting handles POST /i/{path}/caldav/link-existing —
+// the per-item picker for sharing an existing CalDAV list across
+// multiple projax items. Re-runs ListCalendars to validate that the
+// submitted URL is genuinely discoverable (defence against a crafted
+// form pointing at an arbitrary URL), then inserts the item_link.
+func (s *Server) handleCalDAVLinkExisting(w http.ResponseWriter, r *http.Request, path string) {
+	if s.CalDAV == nil {
+		http.Error(w, "caldav not configured", http.StatusServiceUnavailable)
+		return
+	}
+	it, err := s.Store.GetByPath(r.Context(), path)
+	if err != nil {
+		s.fail(w, r, err)
+		return
+	}
+	if err := r.ParseForm(); err != nil {
+		s.fail(w, r, err)
+		return
+	}
+	calURL := strings.TrimSpace(r.FormValue("calendar_url"))
+	if calURL == "" {
+		http.Error(w, "calendar_url required", http.StatusBadRequest)
+		return
+	}
+	// Validate the URL is in the discoverable set — a malicious form must
+	// not be able to seed an item_link pointing at arbitrary HTTP servers.
+	cals, err := s.CalDAV.Client.ListCalendars(r.Context())
+	if err != nil {
+		s.fail(w, r, err)
+		return
+	}
+	var matched *caldav.Calendar
+	for i := range cals {
+		if cals[i].URL == calURL {
+			matched = &cals[i]
+			break
+		}
+	}
+	if matched == nil {
+		http.Error(w, "calendar not in discoverable set", http.StatusBadRequest)
+		return
+	}
+	meta := map[string]any{
+		"display_name":   matched.DisplayName,
+		"calendar_color": matched.Color,
+		"linked_at":      time.Now().UTC().Format(time.RFC3339),
+	}
+	if _, err := s.Store.AddLink(r.Context(), it.ID, refTypeCalDAV, calURL, "contains", meta); err != nil {
+		s.fail(w, r, err)
+		return
+	}
+	http.Redirect(w, r, "/i/"+it.PrimaryPath(), http.StatusSeeOther)
+}
+
 // handleCalDAVCreate handles POST /i/{path}/caldav/create — MKCALENDAR on
 // dav.msbls.de derived from the item slug, then the item_link insert.
 func (s *Server) handleCalDAVCreate(w http.ResponseWriter, r *http.Request, path string) {
@@ -231,6 +318,22 @@ func (s *Server) detailTodos(ctx context.Context, item *store.Item) ([]calendarT
 			s.Logger.Warn("caldav todos", "calendar", l.RefID, "err", err)
 			continue
 		}
+		// Phase 5j per-item filter: when the linked list contains ANY
+		// projax-tagged VTODO it's a managed list — narrow to entries
+		// carrying this item's `projax:<path>` tag. A list with zero
+		// projax tags is a legacy/unmanaged list and renders unfiltered
+		// (existing pre-5j behaviour, untouched). The cutoff still
+		// applies to DoneRecent on the post-filter slice.
+		if caldav.AnyTodoHasProjaxTag(todos) {
+			want := item.PrimaryPath()
+			filtered := todos[:0:0]
+			for _, td := range todos {
+				if caldav.HasProjaxTagFor(td, want) {
+					filtered = append(filtered, td)
+				}
+			}
+			todos = filtered
+		}
 		ct := calendarTasks{
 			CalendarURL: l.RefID,
 			DisplayName: linkDisplay(l),
@@ -310,7 +413,14 @@ func (s *Server) handleCalDAVTodoAction(w http.ResponseWriter, r *http.Request,
 			banner = "Cannot create task with empty summary."
 			break
 		}
-		edit := caldav.VTodoEdit{Summary: &summary}
+		// Phase 5j tag-on-create: every VTODO created from a per-item Add
+		// form gets `projax:<primary-path>` in CATEGORIES so multiple
+		// projax items can share one CalDAV list and the per-item filter
+		// only surfaces the right ones.
+		edit := caldav.VTodoEdit{
+			Summary:    &summary,
+			Categories: []string{caldav.ProjaxCategoryFor(it.PrimaryPath())},
+		}
 		if dueStr := strings.TrimSpace(r.FormValue("due")); dueStr != "" {
 			if t, ok := parseDueInput(dueStr); ok {
 				edit.Due = &t
@@ -426,9 +536,25 @@ func (s *Server) renderTasksSection(w http.ResponseWriter, r *http.Request, it *
 		s.fail(w, r, err)
 		return
 	}
+	// HTMX swaps re-render the section in place; the picker needs the same
+	// AvailableCalendars data the full /i/{path} render computes. Errors
+	// here are non-fatal — degrade to an empty picker.
+	var available []caldav.Calendar
+	if s.CalDAV != nil {
+		caldavLinks, lerr := s.Store.LinksByType(r.Context(), it.ID, refTypeCalDAV)
+		if lerr != nil {
+			s.Logger.Warn("tasks-section caldav links", "path", it.PrimaryPath(), "err", lerr)
+		}
+		acs, aerr := s.availableCalendarsForItem(r.Context(), caldavLinks)
+		if aerr != nil {
+			s.Logger.Warn("tasks-section available caldav", "path", it.PrimaryPath(), "err", aerr)
+		}
+		available = acs
+	}
 	data := map[string]any{
 		"Item":               it,
 		"Tasks":              tasks,
+		"AvailableCalendars": available,
 		"CalDAVOn":           s.CalDAV != nil,
 		"Banner":             banner,
 	}

web/caldav_link_existing_test.go

@@ -0,0 +1,419 @@
+package web_test
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"github.com/m/projax/caldav"
+	"github.com/m/projax/web"
+)
+
+// fakeCalDAVServer is a minimal in-memory CalDAV server: a PROPFIND on
+// /dav/calendars/m/ returns a fixed two-calendar list, REPORT on each
+// calendar returns whichever VTODOs the test seeded into todos[url],
+// and PUT to a calendar URL captures the body so the test can assert
+// on what projax wrote. Mirrors the pattern in dashboard_events_test.go
+// but tailored to the Phase 5j flows.
+type fakeCalDAVServer struct {
+	mu         sync.Mutex
+	srv        *httptest.Server
+	calendars  []caldav.Calendar
+	todos      map[string][]string // calendarURL → list of VTODO ICS docs
+	puts       map[string]string   // url → body of the latest PUT to that url
+}
+
+func newFakeCalDAVServer(t *testing.T, cals []caldav.Calendar) *fakeCalDAVServer {
+	t.Helper()
+	f := &fakeCalDAVServer{
+		todos: map[string][]string{},
+		puts:  map[string]string{},
+	}
+	mux := http.NewServeMux()
+	mux.HandleFunc("/dav/calendars/m/", func(w http.ResponseWriter, r *http.Request) {
+		if r.Method == "PROPFIND" {
+			f.mu.Lock()
+			cs := f.calendars
+			f.mu.Unlock()
+			w.WriteHeader(207)
+			_, _ = io.WriteString(w, propfindMultistatus(cs))
+			return
+		}
+		http.Error(w, "method "+r.Method, http.StatusMethodNotAllowed)
+	})
+	// Per-calendar handler. Keyed by URL PATH so both the registration
+	// loop and the test's seed lookup (`fake.todos[calURL]`) resolve to
+	// the same map entry regardless of how the httptest host gets baked
+	// into the full URL.
+	for _, c := range cals {
+		path := urlPathOf(c.URL)
+		mux.HandleFunc(path, func(w http.ResponseWriter, r *http.Request) {
+			switch r.Method {
+			case "REPORT":
+				f.mu.Lock()
+				body := buildReportMultistatus(path, f.todos[path])
+				f.mu.Unlock()
+				w.WriteHeader(207)
+				_, _ = io.WriteString(w, body)
+			case "PUT":
+				body, _ := io.ReadAll(r.Body)
+				f.mu.Lock()
+				f.puts[r.URL.String()] = string(body)
+				f.todos[path] = append(f.todos[path], string(body))
+				f.mu.Unlock()
+				w.Header().Set("ETag", `"fresh"`)
+				w.WriteHeader(http.StatusCreated)
+			default:
+				http.Error(w, "method "+r.Method, http.StatusMethodNotAllowed)
+			}
+		})
+	}
+	f.srv = httptest.NewServer(mux)
+	f.calendars = make([]caldav.Calendar, len(cals))
+	// Rewrite URLs to point at the httptest server's host.
+	for i, c := range cals {
+		f.calendars[i] = caldav.Calendar{
+			URL:         f.srv.URL + urlPathOf(c.URL),
+			HRef:        urlPathOf(c.URL),
+			DisplayName: c.DisplayName,
+			Color:       c.Color,
+		}
+	}
+	t.Cleanup(f.srv.Close)
+	return f
+}
+
+func urlPathOf(absURL string) string {
+	u, _ := url.Parse(absURL)
+	return u.Path
+}
+
+// propfindMultistatus builds the PROPFIND response for the slice of
+// calendars. Includes the collection itself + each calendar entry, plus
+// an "inbox" non-calendar that ListCalendars must filter out.
+func propfindMultistatus(cals []caldav.Calendar) string {
+	var b strings.Builder
+	b.WriteString(`<?xml version="1.0"?><d:multistatus xmlns:d="DAV:" xmlns:cal="urn:ietf:params:xml:ns:caldav">`)
+	b.WriteString(`<d:response><d:href>/dav/calendars/m/</d:href><d:propstat><d:prop><d:resourcetype><d:collection/></d:resourcetype></d:prop><d:status>HTTP/1.1 200 OK</d:status></d:propstat></d:response>`)
+	for _, c := range cals {
+		b.WriteString(`<d:response><d:href>` + urlPathOf(c.URL) + `</d:href><d:propstat><d:prop><d:displayname>` + c.DisplayName + `</d:displayname><d:resourcetype><d:collection/><cal:calendar/></d:resourcetype></d:prop><d:status>HTTP/1.1 200 OK</d:status></d:propstat></d:response>`)
+	}
+	b.WriteString(`</d:multistatus>`)
+	return b.String()
+}
+
+// buildReportMultistatus wraps a slice of VTODO ICS docs into a REPORT
+// multistatus body, one <d:response> per VTODO.
+func buildReportMultistatus(calPath string, vtodos []string) string {
+	if len(vtodos) == 0 {
+		return `<?xml version="1.0"?><d:multistatus xmlns:d="DAV:" xmlns:cal="urn:ietf:params:xml:ns:caldav"></d:multistatus>`
+	}
+	var b strings.Builder
+	b.WriteString(`<?xml version="1.0"?><d:multistatus xmlns:d="DAV:" xmlns:cal="urn:ietf:params:xml:ns:caldav">`)
+	for i, ics := range vtodos {
+		b.WriteString(`<d:response><d:href>` + calPath + "t" + itoa(i) + `.ics</d:href><d:propstat><d:prop><d:getetag>"e` + itoa(i) + `"</d:getetag><cal:calendar-data>`)
+		b.WriteString(ics)
+		b.WriteString(`</cal:calendar-data></d:prop><d:status>HTTP/1.1 200 OK</d:status></d:propstat></d:response>`)
+	}
+	b.WriteString(`</d:multistatus>`)
+	return b.String()
+}
+
+func itoa(n int) string {
+	if n == 0 {
+		return "0"
+	}
+	var buf [20]byte
+	i := len(buf)
+	neg := false
+	if n < 0 {
+		neg = true
+		n = -n
+	}
+	for n > 0 {
+		i--
+		buf[i] = byte('0' + n%10)
+		n /= 10
+	}
+	if neg {
+		i--
+		buf[i] = '-'
+	}
+	return string(buf[i:])
+}
+
+// seedItemUnderDev inserts a fresh projax item under dev and returns
+// its id + primary path. Callers defer cleanup.
+func seedItemUnderDev(t *testing.T, pool *pgxpool.Pool, slug, title string) (id, primaryPath string) {
+	t.Helper()
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+	var dev string
+	if err := pool.QueryRow(ctx, `select id from projax.items where slug='dev' and cardinality(parent_ids)=0`).Scan(&dev); err != nil {
+		t.Fatalf("dev: %v", err)
+	}
+	if err := pool.QueryRow(ctx,
+		`insert into projax.items (kind, title, slug, parent_ids)
+		  values (array['project']::text[], $1, $2, ARRAY[$3]::uuid[])
+		  returning id`,
+		title, slug, dev,
+	).Scan(&id); err != nil {
+		t.Fatalf("seed item: %v", err)
+	}
+	return id, "dev." + slug
+}
+
+// TestDetailLinkExistingCalendar walks the original ask end-to-end:
+// 1. Fake CalDAV server exposes 3 calendars + zero VTODOs.
+// 2. Seed an unlinked projax item under dev.
+// 3. GET /i/{path} — assert the "link existing" <select> renders with
+//    all 3 calendars.
+// 4. POST /i/{path}/caldav/link-existing with one URL.
+// 5. GET /i/{path} again — assert the linked URL is gone from the
+//    picker (already linked) but appears in the tasks section.
+func TestDetailLinkExistingCalendar(t *testing.T) {
+	srv, pool := mustServer(t)
+	defer pool.Close()
+
+	cals := []caldav.Calendar{
+		{URL: "https://dav.test/dav/calendars/m/Family/", DisplayName: "Family"},
+		{URL: "https://dav.test/dav/calendars/m/Travel/", DisplayName: "Travel"},
+		{URL: "https://dav.test/dav/calendars/m/Vacations-2026/", DisplayName: "Vacations 2026"},
+	}
+	fake := newFakeCalDAVServer(t, cals)
+	srv.CalDAV = &web.CalDAVDeps{Client: caldav.New(fake.srv.URL+"/dav/calendars/m/", "u", "p")}
+
+	stamp := strings.ReplaceAll(time.Now().UTC().Format("150405.000000"), ".", "")
+	slug := "caldav-link-" + stamp
+	id, primary := seedItemUnderDev(t, pool, slug, "Caldav link test")
+	defer pool.Exec(context.Background(), `delete from projax.items where id=$1`, id)
+
+	h := srv.Routes()
+
+	// Step 3: picker renders with three calendars.
+	_, body := get(t, h, "/i/"+primary)
+	for _, want := range []string{
+		`action="/i/` + primary + `/caldav/link-existing"`,
+		`>Family<`,
+		`>Travel<`,
+		`>Vacations 2026<`,
+		`+ Create new list`,
+	} {
+		if !strings.Contains(body, want) {
+			t.Errorf("unlinked detail page missing %q", want)
+		}
+	}
+
+	// Step 4: POST link-existing. Pick the Vacations 2026 calendar.
+	pickedURL := fake.calendars[2].URL
+	form := url.Values{"calendar_url": {pickedURL}}
+	resp, _ := post(t, h, "/i/"+primary+"/caldav/link-existing", form)
+	if resp != http.StatusSeeOther {
+		t.Fatalf("link-existing POST → %d, want 303", resp)
+	}
+	defer pool.Exec(context.Background(), `delete from projax.item_links where item_id=$1 and ref_id=$2`, id, pickedURL)
+
+	// Step 5: picker no longer offers Vacations 2026 (already linked);
+	// the tasks section now shows the linked calendar's block.
+	_, body = get(t, h, "/i/"+primary)
+	if strings.Contains(body, `<option value="`+pickedURL+`">Vacations 2026</option>`) {
+		t.Errorf("picker should NOT offer the already-linked Vacations 2026 URL")
+	}
+	if !strings.Contains(body, "Vacations 2026") {
+		t.Errorf("tasks section should display the linked Vacations 2026 list")
+	}
+	if !strings.Contains(body, `data-cal="`+pickedURL+`"`) {
+		t.Errorf("tasks section missing cal-block for the linked URL")
+	}
+}
+
+// TestVTodoCreateAttachesProjaxCategory exercises the tag-on-create
+// half of Phase 5j. Posting the Add-task form from /i/{path} must send
+// a VTODO whose CATEGORIES contains `projax:<path>` so a shared list
+// can later be filtered per-item.
+func TestVTodoCreateAttachesProjaxCategory(t *testing.T) {
+	srv, pool := mustServer(t)
+	defer pool.Close()
+
+	cals := []caldav.Calendar{
+		{URL: "https://dav.test/dav/calendars/m/Shared/", DisplayName: "Shared"},
+	}
+	fake := newFakeCalDAVServer(t, cals)
+	srv.CalDAV = &web.CalDAVDeps{Client: caldav.New(fake.srv.URL+"/dav/calendars/m/", "u", "p")}
+
+	stamp := strings.ReplaceAll(time.Now().UTC().Format("150405.000000"), ".", "")
+	slug := "caldav-tag-" + stamp
+	id, primary := seedItemUnderDev(t, pool, slug, "Tag-on-create test")
+	defer pool.Exec(context.Background(), `delete from projax.items where id=$1`, id)
+	calURL := fake.calendars[0].URL
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	if _, err := pool.Exec(ctx,
+		`insert into projax.item_links (item_id, ref_type, ref_id, rel)
+		  values ($1, 'caldav-list', $2, 'contains')`,
+		id, calURL,
+	); err != nil {
+		t.Fatalf("seed link: %v", err)
+	}
+	defer pool.Exec(context.Background(), `delete from projax.item_links where item_id=$1`, id)
+
+	h := srv.Routes()
+	form := url.Values{
+		"calendar_url": {calURL},
+		"summary":      {"Buy travel gear"},
+	}
+	resp, _ := post(t, h, "/i/"+primary+"/caldav/todo/todo-create", form)
+	if resp != http.StatusSeeOther && resp != http.StatusOK {
+		t.Fatalf("todo-create POST → %d", resp)
+	}
+
+	// Inspect what the fake CalDAV server received.
+	fake.mu.Lock()
+	defer fake.mu.Unlock()
+	if len(fake.puts) == 0 {
+		t.Fatalf("expected at least one PUT to the fake CalDAV server")
+	}
+	var got string
+	for _, body := range fake.puts {
+		got = body
+		break
+	}
+	wantTag := "projax:" + primary
+	if !strings.Contains(got, "CATEGORIES:"+wantTag) {
+		t.Errorf("PUT body missing CATEGORIES tag %q. Body:\n%s", wantTag, got)
+	}
+}
+
+// TestDetailFilterByProjaxCategory exercises the read-side filter:
+// when the linked list has ANY projax: tag, the detail page only shows
+// the VTODOs whose CATEGORIES include THIS item's tag. VTODOs tagged
+// for OTHER items must NOT leak through.
+func TestDetailFilterByProjaxCategory(t *testing.T) {
+	srv, pool := mustServer(t)
+	defer pool.Close()
+
+	cals := []caldav.Calendar{
+		{URL: "https://dav.test/dav/calendars/m/Vacations-2026/", DisplayName: "Vacations 2026"},
+	}
+	fake := newFakeCalDAVServer(t, cals)
+	srv.CalDAV = &web.CalDAVDeps{Client: caldav.New(fake.srv.URL+"/dav/calendars/m/", "u", "p")}
+	calURL := fake.calendars[0].URL
+
+	stamp := strings.ReplaceAll(time.Now().UTC().Format("150405.000000"), ".", "")
+	idA, primaryA := seedItemUnderDev(t, pool, "trip-a-"+stamp, "Trip A")
+	idB, primaryB := seedItemUnderDev(t, pool, "trip-b-"+stamp, "Trip B")
+	defer pool.Exec(context.Background(), `delete from projax.items where id in ($1, $2)`, idA, idB)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	for _, id := range []string{idA, idB} {
+		if _, err := pool.Exec(ctx,
+			`insert into projax.item_links (item_id, ref_type, ref_id, rel)
+			  values ($1, 'caldav-list', $2, 'contains')`,
+			id, calURL,
+		); err != nil {
+			t.Fatalf("seed link: %v", err)
+		}
+	}
+	defer pool.Exec(context.Background(), `delete from projax.item_links where ref_id=$1`, calURL)
+
+	// Three VTODOs on the SHARED list: one tagged for A, one for B, one
+	// for both.
+	tagA := "projax:" + primaryA
+	tagB := "projax:" + primaryB
+	fake.mu.Lock()
+	fake.todos[urlPathOf(calURL)] = []string{
+		todoICS("uid-only-a", "Book flight A", []string{tagA}),
+		todoICS("uid-only-b", "Book flight B", []string{tagB}),
+		todoICS("uid-shared", "Travel insurance", []string{tagA, tagB}),
+	}
+	fake.mu.Unlock()
+
+	h := srv.Routes()
+	_, body := get(t, h, "/i/"+primaryA)
+	if !strings.Contains(body, "Book flight A") {
+		t.Errorf("Trip A detail missing tagged-A summary")
+	}
+	if strings.Contains(body, "Book flight B") {
+		t.Errorf("Trip A detail leaked tagged-B summary — filter broken")
+	}
+	if !strings.Contains(body, "Travel insurance") {
+		t.Errorf("Trip A detail missing dual-tagged summary (multi-tag contract)")
+	}
+
+	// Trip B sees the mirror image: B + shared, not A.
+	_, body = get(t, h, "/i/"+primaryB)
+	if strings.Contains(body, "Book flight A") {
+		t.Errorf("Trip B detail leaked tagged-A summary")
+	}
+	if !strings.Contains(body, "Book flight B") {
+		t.Errorf("Trip B detail missing tagged-B summary")
+	}
+	if !strings.Contains(body, "Travel insurance") {
+		t.Errorf("Trip B detail missing dual-tagged summary")
+	}
+}
+
+// TestDetailUntaggedListShowsAll proves the legacy fallback: a linked
+// list with ZERO projax: tags is treated as unmanaged — every VTODO
+// renders, untouched. Without this users with pre-5j lists would see
+// the detail page suddenly hide all their existing tasks.
+func TestDetailUntaggedListShowsAll(t *testing.T) {
+	srv, pool := mustServer(t)
+	defer pool.Close()
+
+	cals := []caldav.Calendar{
+		{URL: "https://dav.test/dav/calendars/m/Home/", DisplayName: "Home"},
+	}
+	fake := newFakeCalDAVServer(t, cals)
+	srv.CalDAV = &web.CalDAVDeps{Client: caldav.New(fake.srv.URL+"/dav/calendars/m/", "u", "p")}
+	calURL := fake.calendars[0].URL
+
+	stamp := strings.ReplaceAll(time.Now().UTC().Format("150405.000000"), ".", "")
+	id, primary := seedItemUnderDev(t, pool, "home-legacy-"+stamp, "Home legacy")
+	defer pool.Exec(context.Background(), `delete from projax.items where id=$1`, id)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	if _, err := pool.Exec(ctx,
+		`insert into projax.item_links (item_id, ref_type, ref_id, rel)
+		  values ($1, 'caldav-list', $2, 'contains')`,
+		id, calURL,
+	); err != nil {
+		t.Fatalf("seed link: %v", err)
+	}
+	defer pool.Exec(context.Background(), `delete from projax.item_links where item_id=$1`, id)
+
+	fake.mu.Lock()
+	fake.todos[urlPathOf(calURL)] = []string{
+		todoICS("legacy-1", "Pick up bread", nil),
+		todoICS("legacy-2", "Call dentist", []string{"home", "errands"}),
+	}
+	fake.mu.Unlock()
+
+	h := srv.Routes()
+	_, body := get(t, h, "/i/"+primary)
+	if !strings.Contains(body, "Pick up bread") {
+		t.Errorf("untagged-list detail missing legacy todo 'Pick up bread'")
+	}
+	if !strings.Contains(body, "Call dentist") {
+		t.Errorf("untagged-list detail missing legacy todo with non-projax categories")
+	}
+}
+
+// todoICS builds a minimal VTODO ICS doc with optional CATEGORIES.
+func todoICS(uid, summary string, categories []string) string {
+	cat := ""
+	if len(categories) > 0 {
+		cat = "CATEGORIES:" + strings.Join(categories, ",") + "\r\n"
+	}
+	return "BEGIN:VCALENDAR\r\nVERSION:2.0\r\nBEGIN:VTODO\r\nUID:" + uid + "\r\nSUMMARY:" + summary + "\r\nSTATUS:NEEDS-ACTION\r\n" + cat + "END:VTODO\r\nEND:VCALENDAR"
+}

web/server.go

@@ -15,6 +15,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/m/projax/caldav"
 	"github.com/m/projax/internal/aggregate"
 	"github.com/m/projax/internal/cache"
 	"github.com/m/projax/internal/itemwrite"
@@ -572,6 +573,22 @@ func (s *Server) handleDetail(w http.ResponseWriter, r *http.Request) {
 	if err != nil {
 		s.Logger.Warn("detail tasks", "path", it.PrimaryPath(), "err", err)
 	}
+	// Phase 5j: pre-load discoverable CalDAV calendars (minus the ones
+	// already linked) so the per-item Tasks section can offer a "Link
+	// existing list" picker alongside the create-new affordance. Errors
+	// are non-fatal — the section falls back to its pre-5j shape.
+	var availableCalendars []caldav.Calendar
+	if s.CalDAV != nil {
+		caldavLinks, lerr := s.Store.LinksByType(r.Context(), it.ID, refTypeCalDAV)
+		if lerr != nil {
+			s.Logger.Warn("detail caldav links", "path", it.PrimaryPath(), "err", lerr)
+		}
+		acs, aerr := s.availableCalendarsForItem(r.Context(), caldavLinks)
+		if aerr != nil {
+			s.Logger.Warn("detail available caldav", "path", it.PrimaryPath(), "err", aerr)
+		}
+		availableCalendars = acs
+	}
 	issues, err := s.detailIssues(r.Context(), it)
 	if err != nil {
 		s.Logger.Warn("detail issues", "path", it.PrimaryPath(), "err", err)
@@ -591,6 +608,7 @@ func (s *Server) handleDetail(w http.ResponseWriter, r *http.Request) {
 		"ParentOptions":    parents,
 		"StatusOptions":    []string{"active", "done", "archived"},
 		"Tasks":              tasks,
+		"AvailableCalendars": availableCalendars,
 		"CalDAVOn":           s.CalDAV != nil,
 		"Issues":             issues,
 		"IssuesOpenTotal":  openTotal,
@@ -610,6 +628,10 @@ func (s *Server) handleDetailWrite(w http.ResponseWriter, r *http.Request) {
 		s.handleCalDAVCreate(w, r, base)
 		return
 	}
+	if base, ok := strings.CutSuffix(path, "/caldav/link-existing"); ok {
+		s.handleCalDAVLinkExisting(w, r, base)
+		return
+	}
 	for _, action := range []string{"complete", "reopen", "edit", "delete", "todo-create"} {
 		if base, ok := strings.CutSuffix(path, "/caldav/todo/"+action); ok {
 			s.handleCalDAVTodoAction(w, r, base, action)

web/templates/tasks_section.tmpl

@@ -94,9 +94,29 @@
     {{end}}
   {{else}}
     <p class="muted">No CalDAV list linked.</p>
-    <form method="post" action="/i/{{.Item.PrimaryPath}}/caldav/create" class="inline">
-      <button type="submit">Create CalDAV list</button>
+  {{end}}
+
+  {{/* Phase 5j: per-item picker for sharing an existing list across
+       multiple projax items (e.g. one "Vacations 2026" list under
+       several admin.vacations sub-items). Renders in BOTH states:
+       unlinked items see it next to Create-new; already-linked items
+       see it as "+ link another" for the multi-list flow. */}}
+  <div class="caldav-actions">
+    {{if .AvailableCalendars}}
+      <form method="post" action="/i/{{.Item.PrimaryPath}}/caldav/link-existing" class="caldav-link-existing inline">
+        <label class="visually-hidden" for="caldav-link-existing-select">Link existing CalDAV list</label>
+        <select id="caldav-link-existing-select" name="calendar_url" required>
+          <option value="">— link existing list —</option>
+          {{range .AvailableCalendars}}<option value="{{.URL}}">{{.DisplayName}}</option>{{end}}
+        </select>
+        <button type="submit">Link</button>
       </form>
     {{end}}
+    {{if not .Tasks}}
+      <form method="post" action="/i/{{.Item.PrimaryPath}}/caldav/create" class="inline">
+        <button type="submit">+ Create new list</button>
+      </form>
+    {{end}}
+  </div>
 </section>
 {{end}}