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/deadlines.go

@@ -20,6 +20,33 @@ func NewDeadlineHandlers(ds *services.DeadlineService, db *sqlx.DB) *DeadlineHan
 	return &DeadlineHandlers{deadlines: ds, db: db}
 }
 
+// Get handles GET /api/deadlines/{deadlineID}
+func (h *DeadlineHandlers) Get(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.GetByID(tenantID, deadlineID)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to fetch deadline")
+		return
+	}
+	if deadline == nil {
+		writeError(w, http.StatusNotFound, "deadline not found")
+		return
+	}
+
+	writeJSON(w, http.StatusOK, deadline)
+}
+
 // ListAll handles GET /api/deadlines
 func (h *DeadlineHandlers) ListAll(w http.ResponseWriter, r *http.Request) {
 	tenantID, err := resolveTenant(r, h.db)