test(timetable): add real-API contract fixtures as schema regression test (3 shape variants) (B6)

apps/app/tests/unit/schemas/timetableContractShape.test.ts (NEW, 5 tests):
  - base shape: one performance with stage assigned + full engagement
    (Bert's browser-tested sample, field-for-field). Asserts decimal-as-
    string contract on fee_amount/buma_percentage/vat_percentage AND
    enum-label wrapper on booking_status AND nested computed object.
  - parked shape: stage_id=null, stage=null (Wachtrij case)
  - multi-perf shape: two performances sharing engagement_id
    (RFC §D17 "Friday + Saturday under one combined deal")
  - sanity: individual performanceSchema parses each fixture element
  - regression guard: a payload with NUMBER fee_amount throws (locks
    out the pre-B5 bug class)

Every fixture spells out explicit `null` for the schema's nullable-but-
required fields (timestamps, notes, deal_breakdown) so the
nullable() vs optional() distinction is exercised, not glossed over.

Schema surface change to support the test:
  apps/app/src/schemas/timetable.ts now EXPORTS performanceArraySchema
  (previously a private const inside useTimetable.ts).
  apps/app/src/composables/api/useTimetable.ts imports the shared one
  instead of redeclaring it locally — single source of truth for the
  array shape consumers and tests share.

Test count: 397 → 402 (+5). Typecheck clean.

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

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

@@ -5,7 +5,7 @@ import { z } from 'zod'
 import { apiClient } from '@/lib/axios'
 import {
   artistEngagementSchema,
-  performanceSchema,
+  performanceArraySchema,
   stageSchema,
 } from '@/schemas/timetable'
 import type {
@@ -15,7 +15,6 @@ import type {
 } from '@/types/timetable'
 
 const stageArraySchema = z.array(stageSchema)
-const performanceArraySchema = z.array(performanceSchema)
 
 /**
  * RFC v0.2 §6.2 — read-side composables for the timetable canvas.