feat: UPC deadline determination — event-driven model with proceeding timeline

Full event-driven deadline determination system ported from youpc.org:

Backend:
- DetermineService: walks proceeding event tree, calculates cascading
  dates with holiday adjustment and conditional logic
- GET /api/proceeding-types/{code}/timeline — full event tree structure
- POST /api/deadlines/determine — calculate timeline with conditions
- POST /api/cases/{caseID}/deadlines/batch — batch-create deadlines
- DeadlineRule model: added is_spawn, spawn_label fields
- GetFullTimeline: recursive CTE following cross-type spawn branches
- Conditional deadlines: condition_rule_id toggles alt_duration/rule_code
  (e.g. Reply changes from RoP.029b to RoP.029a when CCR is filed)
- Seed SQL with full UPC event trees (INF, REV, CCR, APM, APP, AMD)

Frontend:
- DeadlineWizard: interactive proceeding timeline with step-by-step flow
  1. Select proceeding type (visual cards)
  2. Enter trigger event date
  3. Toggle conditional branches (CCR, Appeal, Amend)
  4. See full calculated timeline with color-coded urgency
  5. Batch-create all deadlines on a selected case
- Visual timeline tree with party icons, rule codes, duration badges
- Kept existing DeadlineCalculator as "Schnell" quick mode

Also resolved merge conflicts across 6 files (auth, router, handlers)
merging role-based permissions + audit trail features.

backend/internal/handlers/determine.go

@@ -0,0 +1,127 @@
+package handlers
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/google/uuid"
+
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/auth"
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/services"
+)
+
+// DetermineHandlers holds handlers for deadline determination endpoints
+type DetermineHandlers struct {
+	determine *services.DetermineService
+	deadlines *services.DeadlineService
+}
+
+// NewDetermineHandlers creates determine handlers
+func NewDetermineHandlers(determine *services.DetermineService, deadlines *services.DeadlineService) *DetermineHandlers {
+	return &DetermineHandlers{determine: determine, deadlines: deadlines}
+}
+
+// GetTimeline handles GET /api/proceeding-types/{code}/timeline
+// Returns the full event tree for a proceeding type (no date calculations)
+func (h *DetermineHandlers) GetTimeline(w http.ResponseWriter, r *http.Request) {
+	code := r.PathValue("code")
+	if code == "" {
+		writeError(w, http.StatusBadRequest, "proceeding type code required")
+		return
+	}
+
+	timeline, pt, err := h.determine.GetTimeline(code)
+	if err != nil {
+		writeError(w, http.StatusNotFound, "proceeding type not found")
+		return
+	}
+
+	writeJSON(w, http.StatusOK, map[string]any{
+		"proceeding_type": pt,
+		"timeline":        timeline,
+	})
+}
+
+// Determine handles POST /api/deadlines/determine
+// Calculates the full timeline with cascading dates and conditional logic
+func (h *DetermineHandlers) Determine(w http.ResponseWriter, r *http.Request) {
+	var req services.DetermineRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	if req.ProceedingType == "" || req.TriggerEventDate == "" {
+		writeError(w, http.StatusBadRequest, "proceeding_type and trigger_event_date are required")
+		return
+	}
+
+	resp, err := h.determine.Determine(req)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, err.Error())
+		return
+	}
+
+	writeJSON(w, http.StatusOK, resp)
+}
+
+// BatchCreate handles POST /api/cases/{caseID}/deadlines/batch
+// Creates multiple deadlines on a case from determined timeline
+func (h *DetermineHandlers) BatchCreate(w http.ResponseWriter, r *http.Request) {
+	tenantID, ok := auth.TenantFromContext(r.Context())
+	if !ok {
+		writeError(w, http.StatusForbidden, "missing tenant")
+		return
+	}
+
+	caseID, err := parsePathUUID(r, "caseID")
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid case ID")
+		return
+	}
+
+	var req struct {
+		Deadlines []struct {
+			Title           string     `json:"title"`
+			DueDate         string     `json:"due_date"`
+			OriginalDueDate *string    `json:"original_due_date,omitempty"`
+			RuleID          *uuid.UUID `json:"rule_id,omitempty"`
+			RuleCode        *string    `json:"rule_code,omitempty"`
+			Notes           *string    `json:"notes,omitempty"`
+		} `json:"deadlines"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	if len(req.Deadlines) == 0 {
+		writeError(w, http.StatusBadRequest, "at least one deadline is required")
+		return
+	}
+
+	var created int
+	for _, d := range req.Deadlines {
+		if d.Title == "" || d.DueDate == "" {
+			continue
+		}
+		input := services.CreateDeadlineInput{
+			CaseID:  caseID,
+			Title:   d.Title,
+			DueDate: d.DueDate,
+			Source:   "determined",
+			RuleID:  d.RuleID,
+			Notes:   d.Notes,
+		}
+		_, err := h.deadlines.Create(r.Context(), tenantID, input)
+		if err != nil {
+			internalError(w, "failed to create deadline", err)
+			return
+		}
+		created++
+	}
+
+	writeJSON(w, http.StatusCreated, map[string]any{
+		"created": created,
+	})
+}