test(timetable): useTimetableMutations 409 rollback + idempotency-key semantics (Step 9)

Minimal seam in src/composables/api/useTimetableMutations.ts: the move()
mutation's onError now calls useNotificationStore().show(...) on a 409
status. Generic axios errors stay quiet here — the global response
handler in lib/axios/factory.ts already toasts those. RFC D14 wanted
the version-mismatch toast specifically.

apps/app/tests/component/useTimetableMutations.test.ts (NEW, 5 tests):
  - on success: returns server payload with bumped version + sends the
    Idempotency-Key supplied by the caller
  - 409: rejects with VersionMismatchError + notification.show()
    invoked once with the Dutch translation + 'error' level
  - cascade: success with cascaded[] populated puts those peers into
    the result.cascaded array
  - Idempotency-Key uniqueness: two distinct logical move() calls send
    distinct keys
  - Idempotency-Key reuse: caller-controlled retry within the same
    logical action sends the SAME key on the wire (so the backend's
    60s idempotency middleware dedupes)

The two existing unit-project tests now register a Pinia instance
(createPinia + setActivePinia) so useNotificationStore() resolves.
Existing assertions unchanged.

Test count: 364 → 369.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/app/src/composables/api/useTimetableMutations.ts

@@ -10,6 +10,7 @@ import {
   performanceSchema,
   stageSchema,
 } from '@/schemas/timetable'
+import { useNotificationStore } from '@/stores/useNotificationStore'
 import type {
   CreatePerformancePayload,
   CreateStagePayload,
@@ -64,6 +65,7 @@ function isVersionMismatch(err: unknown): err is { response: { status: 409; data
 
 export function useTimetableMutations(args: UseTimetableMutationsArgs) {
   const queryClient = useQueryClient()
+  const notification = useNotificationStore()
   const { orgId, eventId, dayId } = args
 
   const performancesKey = () => ['timetable', 'performances', eventId, dayId] as const
@@ -173,13 +175,19 @@ export function useTimetableMutations(args: UseTimetableMutationsArgs) {
       mergePerformance(result.moved)
       applyCascade(result.cascaded)
     },
-    onError: (_err, _vars, ctx) => {
+    onError: (err, _vars, ctx) => {
       // Restore cached blocks from snapshot so the canvas snaps back.
       if (ctx?.snapshot)
         mergePerformance(ctx.snapshot)
       if (ctx?.snapshotWachtrij)
         mergePerformance(ctx.snapshotWachtrij)
       invalidate()
+
+      // RFC D14 — version mismatch toast. Generic axios errors stay quiet
+      // here; they're already surfaced by the global response handler in
+      // lib/axios/factory.ts.
+      if ((err as { status?: number } | null)?.status === 409)
+        notification.show('Iemand anders heeft dit zojuist aangepast — venster ververst.', 'error')
     },
   })