feat: add deadline CRUD, calculator, and holiday services (Phase 1C)

- Holiday service with German federal holidays, Easter calculation, DB loading
- Deadline calculator adapted from youpc.org (duration calc + non-working day adjustment)
- Deadline CRUD service (tenant-scoped: list, create, update, complete, delete)
- Deadline rule service (list, filter by proceeding type, hierarchical rule trees)
- HTTP handlers for all endpoints with tenant resolution via X-Tenant-ID header
- Router wired with all new endpoints under /api/
- Tests for holiday and calculator services (8 passing)

backend/internal/handlers/deadlines.go

@@ -0,0 +1,162 @@
+package handlers
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/jmoiron/sqlx"
+
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/services"
+)
+
+// DeadlineHandlers holds handlers for deadline CRUD endpoints
+type DeadlineHandlers struct {
+	deadlines *services.DeadlineService
+	db        *sqlx.DB
+}
+
+// NewDeadlineHandlers creates deadline handlers
+func NewDeadlineHandlers(ds *services.DeadlineService, db *sqlx.DB) *DeadlineHandlers {
+	return &DeadlineHandlers{deadlines: ds, db: db}
+}
+
+// ListForCase handles GET /api/cases/{caseID}/deadlines
+func (h *DeadlineHandlers) ListForCase(w http.ResponseWriter, r *http.Request) {
+	tenantID, err := resolveTenant(r, h.db)
+	if err != nil {
+		handleTenantError(w, err)
+		return
+	}
+
+	caseID, err := parsePathUUID(r, "caseID")
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid case ID")
+		return
+	}
+
+	deadlines, err := h.deadlines.ListForCase(tenantID, caseID)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to list deadlines")
+		return
+	}
+
+	writeJSON(w, http.StatusOK, deadlines)
+}
+
+// Create handles POST /api/cases/{caseID}/deadlines
+func (h *DeadlineHandlers) Create(w http.ResponseWriter, r *http.Request) {
+	tenantID, err := resolveTenant(r, h.db)
+	if err != nil {
+		handleTenantError(w, err)
+		return
+	}
+
+	caseID, err := parsePathUUID(r, "caseID")
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid case ID")
+		return
+	}
+
+	var input services.CreateDeadlineInput
+	if err := json.NewDecoder(r.Body).Decode(&input); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+	input.CaseID = caseID
+
+	if input.Title == "" || input.DueDate == "" {
+		writeError(w, http.StatusBadRequest, "title and due_date are required")
+		return
+	}
+
+	deadline, err := h.deadlines.Create(tenantID, input)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to create deadline")
+		return
+	}
+
+	writeJSON(w, http.StatusCreated, deadline)
+}
+
+// Update handles PUT /api/deadlines/{deadlineID}
+func (h *DeadlineHandlers) Update(w http.ResponseWriter, r *http.Request) {
+	tenantID, err := resolveTenant(r, h.db)
+	if err != nil {
+		handleTenantError(w, err)
+		return
+	}
+
+	deadlineID, err := parsePathUUID(r, "deadlineID")
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid deadline ID")
+		return
+	}
+
+	var input services.UpdateDeadlineInput
+	if err := json.NewDecoder(r.Body).Decode(&input); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	deadline, err := h.deadlines.Update(tenantID, deadlineID, input)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to update deadline")
+		return
+	}
+	if deadline == nil {
+		writeError(w, http.StatusNotFound, "deadline not found")
+		return
+	}
+
+	writeJSON(w, http.StatusOK, deadline)
+}
+
+// Complete handles PATCH /api/deadlines/{deadlineID}/complete
+func (h *DeadlineHandlers) Complete(w http.ResponseWriter, r *http.Request) {
+	tenantID, err := resolveTenant(r, h.db)
+	if err != nil {
+		handleTenantError(w, err)
+		return
+	}
+
+	deadlineID, err := parsePathUUID(r, "deadlineID")
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid deadline ID")
+		return
+	}
+
+	deadline, err := h.deadlines.Complete(tenantID, deadlineID)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to complete deadline")
+		return
+	}
+	if deadline == nil {
+		writeError(w, http.StatusNotFound, "deadline not found")
+		return
+	}
+
+	writeJSON(w, http.StatusOK, deadline)
+}
+
+// Delete handles DELETE /api/deadlines/{deadlineID}
+func (h *DeadlineHandlers) Delete(w http.ResponseWriter, r *http.Request) {
+	tenantID, err := resolveTenant(r, h.db)
+	if err != nil {
+		handleTenantError(w, err)
+		return
+	}
+
+	deadlineID, err := parsePathUUID(r, "deadlineID")
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid deadline ID")
+		return
+	}
+
+	err = h.deadlines.Delete(tenantID, deadlineID)
+	if err != nil {
+		writeError(w, http.StatusNotFound, err.Error())
+		return
+	}
+
+	writeJSON(w, http.StatusOK, map[string]string{"status": "deleted"})
+}