feat: Phase A backend — notes CRUD, detail endpoints, dashboard fix

- Create kanzlai.notes table (polymorphic FK with CHECK constraint,
  partial indexes, RLS)
- Add Note model, NoteService (ListByParent, Create, Update, Delete),
  and NoteHandler with endpoints: GET/POST /api/notes, PUT/DELETE /api/notes/{id}
- Add GET /api/deadlines/{deadlineID} detail endpoint
- Add GET /api/appointments/{id} detail endpoint
- Add GET /api/case-events/{id} detail endpoint (new CaseEventHandler)
- Fix dashboard query: add case_id to upcoming_deadlines SELECT,
  add id and case_id to recent_activity SELECT
- Register all new routes in router.go

backend/internal/handlers/case_events.go

@@ -0,0 +1,52 @@
+package handlers
+
+import (
+	"database/sql"
+	"errors"
+	"net/http"
+
+	"github.com/google/uuid"
+
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/auth"
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/models"
+	"github.com/jmoiron/sqlx"
+)
+
+type CaseEventHandler struct {
+	db *sqlx.DB
+}
+
+func NewCaseEventHandler(db *sqlx.DB) *CaseEventHandler {
+	return &CaseEventHandler{db: db}
+}
+
+// Get handles GET /api/case-events/{id}
+func (h *CaseEventHandler) Get(w http.ResponseWriter, r *http.Request) {
+	tenantID, ok := auth.TenantFromContext(r.Context())
+	if !ok {
+		writeError(w, http.StatusUnauthorized, "missing tenant")
+		return
+	}
+
+	eventID, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid event ID")
+		return
+	}
+
+	var event models.CaseEvent
+	err = h.db.GetContext(r.Context(), &event,
+		`SELECT id, tenant_id, case_id, event_type, title, description, event_date, created_by, metadata, created_at, updated_at
+		 FROM case_events
+		 WHERE id = $1 AND tenant_id = $2`, eventID, tenantID)
+	if err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			writeError(w, http.StatusNotFound, "case event not found")
+			return
+		}
+		writeError(w, http.StatusInternalServerError, "failed to fetch case event")
+		return
+	}
+
+	writeJSON(w, http.StatusOK, event)
+}