feat(timetable): observers — engagement denorm/guard + performance version bump

ArtistEngagementObserver:
- creating: auto-fills organisation_id from parent Artist (RFC v0.2 D10
  denormalisation), asserts artist.organisation_id == event.organisation_id;
  cross-tenant linkage throws CrossTenantEngagementException (extends
  DomainException, included in this commit).
- saving: no-op marker reserved for Session 2 state-machine validation.
- deleted: cascades soft-delete to Performance children, hard-deletes
  AdvanceSection children. AdvanceSubmission rows are immutable per
  RFC §5.4 and remain attached.

PerformanceObserver:
- saving: increments version by 1 on UPDATE only (D14 optimistic lock).
  MoveTimetablePerformanceRequest in Session 2 uses this for concurrent-
  edit detection.

Both observers registered in AppServiceProvider::boot.

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

api/app/Exceptions/Artist/CrossTenantEngagementException.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\Artist;
+
+use App\Models\ArtistEngagement;
+use DomainException;
+
+/**
+ * Raised when an ArtistEngagement is being created with an artist and
+ * event that belong to different organisations. The engagement's
+ * `organisation_id` is denormalised from the artist (RFC v0.2 D10);
+ * the event must match. Cross-tenant linkage is a hard error — fail
+ * loud rather than silently denormalise the wrong tenant.
+ */
+final class CrossTenantEngagementException extends DomainException
+{
+    public static function forEngagement(ArtistEngagement $engagement): self
+    {
+        return new self(sprintf(
+            'ArtistEngagement cross-tenant: artist=%s (org=%s) vs event=%s (org=%s).',
+            $engagement->artist_id ?? 'null',
+            $engagement->artist?->organisation_id ?? 'null',
+            $engagement->event_id ?? 'null',
+            $engagement->event?->organisation_id ?? 'null',
+        ));
+    }
+}