fix(timetable): align Zod decimal fields with backend wire format (decimal-as-string per Laravel cast) (B5)

Phase A diagnosed the "Kon timetable niet laden" browser symptom as Zod
schema drift. The prompt's hypothesis (enum {value, label} mismatch) was
incorrect — the schema already uses the enumLabel() wrapper for every
enum field. The actual drift is decimal-cast columns: Laravel serialises
`decimal(N,M)` columns as strings to preserve precision, but the schema
expected numbers, so the very first response triggered a ZodError.

Affected fields, all on `artist_engagements`:
  fee_amount         decimal(10,2)  → wire `"11503.58"`, schema was z.number()
  buma_percentage    decimal(5,2)   → wire `"7.00"`,     schema was z.number()
  vat_percentage     decimal(5,2)   → wire `"21.00"`,    schema was z.number()
  deposit_percentage decimal(5,2)   → wire `"…"`,        schema was z.number()

Backend has no explicit `decimal:N` cast on these columns
(api/app/Models/ArtistEngagement.php:64-85 — the `casts()` method covers
the enums + booleans + dates + integers, but skips decimals).

Per the strategic decision (frontend adapts, backend stays):
  - schemas/timetable.ts: four fields → z.string().nullable()
  - types/timetable.ts: matching ArtistEngagement interface fields →
    `string | null`
  - PerformancePopover.vue:129: only consumer doing arithmetic on a
    decimal field; coerce at the use site via Number(...).toFixed(2).
    Single line.
  - tests/component/PerformanceBlock.test.ts + tests/a11y/axe.test.ts:
    spot-checked mocks; the two with hand-built engagement payloads
    flipped fee_amount/buma_percentage/vat_percentage from numbers to
    strings to match the new schema. No other mocks needed updating.

The {value, label} enum wrapper claim in the prompt was specifically
debunked in Phase A — every consumer (Wachtrij, PerformanceBlock,
WachtrijCard, PerformancePopover, AddPerformanceDialog, page entry)
already uses .value/.label access against an enumLabel-wrapped schema.

B6 will lock the wire-format contract with a real-API fixture
regression test.

All 397 tests still pass; typecheck clean.

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

apps/app/src/components/timetable/PerformancePopover.vue

@@ -126,7 +126,7 @@ function close(): void {
         >
           <div class="tt-popover__row">
             <span>Fee</span>
-            <span>€{{ (engagement.fee_amount ?? 0).toFixed(2) }}</span>
+            <span>€{{ Number(engagement.fee_amount ?? 0).toFixed(2) }}</span>
           </div>
           <div class="tt-popover__row">
             <span>Buma</span>

apps/app/src/schemas/timetable.ts

@@ -110,18 +110,23 @@ export const artistEngagementSchema = z.object({
     })
     .optional(),
   booking_status: enumLabel(artistEngagementStatusSchema),
-  fee_amount: z.number().nullable(),
+
+  // Decimal columns serialise as strings on the wire (Laravel default for
+  // `decimal(N,M)` without an explicit cast — preserves precision). The
+  // schema mirrors that; consumers that need arithmetic coerce via
+  // Number()/parseFloat at the use site.
+  fee_amount: z.string().nullable(),
   fee_currency: z.string(),
   fee_type: enumLabel(feeTypeSchema),
   buma_applicable: z.boolean(),
-  buma_percentage: z.number().nullable(),
+  buma_percentage: z.string().nullable(),
   buma_handled_by: enumLabel(bumaHandledBySchema),
   vat_applicable: z.boolean(),
-  vat_percentage: z.number().nullable(),
+  vat_percentage: z.string().nullable(),
   deal_breakdown: z
     .array(z.object({ label: z.string().optional(), amount: z.number() }))
     .nullable(),
-  deposit_percentage: z.number().nullable(),
+  deposit_percentage: z.string().nullable(),
   deposit_due_date: z.string().nullable(),
   balance_due_date: z.string().nullable(),
   payment_status: enumLabel(paymentStatusSchema),

apps/app/src/types/timetable.ts

@@ -133,16 +133,22 @@ export interface ArtistEngagement {
   project_leader_id: string | null
   project_leader?: ArtistEngagementProjectLeader
   booking_status: EnumLabel<ArtistEngagementStatus>
-  fee_amount: number | null
+
+  /**
+   * Decimal-cast columns serialise as strings on the wire (Laravel default
+   * for `decimal(N,M)` without an explicit cast — preserves precision).
+   * Coerce via Number()/parseFloat at consumption sites that need arithmetic.
+   */
+  fee_amount: string | null
   fee_currency: string
   fee_type: EnumLabel<FeeType>
   buma_applicable: boolean
-  buma_percentage: number | null
+  buma_percentage: string | null
   buma_handled_by: EnumLabel<BumaHandledBy>
   vat_applicable: boolean
-  vat_percentage: number | null
+  vat_percentage: string | null
   deal_breakdown: DealBreakdownLine[] | null
-  deposit_percentage: number | null
+  deposit_percentage: string | null
   deposit_due_date: string | null
   balance_due_date: string | null
   payment_status: EnumLabel<PaymentStatus>