feat(timetable): four custom validation rules for artist domain

StageActiveOnEvent — checks the candidate stage_id is linked to the
given event_id via stage_days. Covers performance create/update
(perf.event_id ↔ stage) and the timetable move endpoint
(target_stage_id ↔ resolved target event).

WithinEventBounds — checks a candidate datetime is inside the event's
[start_at, end_at] window. Used for performance start/end dates and
move-target dates against the relevant sub-event for festivals.

OptionExpiresInFuture — conditional rule fired only when
booking_status === 'option'. Asserts option_expires_at is set and in
the future. Implementation of RFC §10.1 transition gate at the
request layer (the service layer enforces the same invariant).

ContractRequiresFee — conditional rule fired only when
booking_status === 'contracted'. Asserts fee_amount is set and > 0.
Same dual-layer enforcement as OptionExpiresInFuture.

All four pass silently when the validated field is null or the
context is irrelevant — the FormRequest still owns the surrounding
required/nullable/exists rules.

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

api/app/Rules/Artist/WithinEventBounds.php

@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Rules\Artist;
+
+use App\Models\Event;
+use Carbon\CarbonImmutable;
+use Closure;
+use Illuminate\Contracts\Validation\ValidationRule;
+
+/**
+ * Validates that a candidate datetime sits inside the event's
+ * [start_at, end_at] window. Works for flat events (single window)
+ * and for sub-events (sub-event window); festival-level events are
+ * usually not the target — the FormRequest passes the *sub-event* id
+ * for performances and the *containing* event for the stage's day.
+ *
+ * Rule passes silently when value is null or when the event cannot
+ * be found (the FormRequest owns the `required`/`exists` decision).
+ */
+final class WithinEventBounds implements ValidationRule
+{
+    public function __construct(
+        private readonly ?string $eventId,
+    ) {}
+
+    public function validate(string $attribute, mixed $value, Closure $fail): void
+    {
+        if ($value === null || $this->eventId === null) {
+            return;
+        }
+
+        $event = Event::withoutGlobalScopes()->find($this->eventId);
+        if ($event === null) {
+            return;
+        }
+
+        $candidate = CarbonImmutable::parse((string) $value);
+        $start = CarbonImmutable::instance($event->start_at);
+        $end = CarbonImmutable::instance($event->end_at);
+
+        if ($candidate->lt($start) || $candidate->gt($end)) {
+            $fail(sprintf(
+                'De datum moet binnen het evenement (%s — %s) vallen.',
+                $start->format('Y-m-d H:i'),
+                $end->format('Y-m-d H:i'),
+            ));
+        }
+    }
+}