feat(timetable): types + zod schemas + idempotency-key helper (Session 4 step 1)

- Extract generateIdempotencyKey() from useFormDraft into reusable lib/
- New types/timetable.ts mirrors PerformanceResource, ArtistEngagementResource,
  StageResource, GenreResource and the four enums verbatim
- New schemas/timetable.ts adds zod parsers for runtime validation of API
  responses + form payloads (createPerformance, createStage, moveTimetable)

RFC v0.2 §10 contract surface for the upcoming timetable canvas.

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

apps/app/src/composables/useFormDraft.ts

@@ -7,6 +7,7 @@ import {
   useSaveFormDraft,
   useSubmitForm,
 } from '@/composables/api/usePublicForm'
+import { generateIdempotencyKey } from '@/lib/idempotencyKey'
 import type { FormValues, PublicFormSubmission, SaveDraftBody } from '@/types/forms/formBuilder'
 
 /** sessionStorage key for reusing an idempotency key across reloads. */
@@ -19,25 +20,6 @@ export function draftSubmitterStorageKey(token: string): string {
   return `draft_submitter:${token}`
 }
 
-function generateIdempotencyKey(): string {
-  const c = (globalThis as { crypto?: { randomUUID?: () => string; getRandomValues?: (arr: Uint8Array) => Uint8Array } }).crypto
-  if (c?.randomUUID) {
-    // UUID v4 (36 chars) exceeds backend max:30. Backend expects 6..30
-    // chars so compress to 24 hex chars (still collision-resistant).
-    return c.randomUUID().replace(/-/g, '').slice(0, 24)
-  }
-  if (c?.getRandomValues) {
-    const buf = new Uint8Array(12)
-
-    c.getRandomValues(buf)
-
-    return Array.from(buf, b => b.toString(16).padStart(2, '0')).join('')
-  }
-
-  // Last-resort fallback — still within 6..30.
-  return `idem-${Date.now().toString(36)}-${Math.random().toString(36).slice(2, 10)}`.slice(0, 30)
-}
-
 interface UseFormDraftOptions {
 
   /** Preferred locale string for `submitted_in_locale` (e.g. `"nl"`). */