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/Observers/PerformanceObserver.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Observers;
+
+use App\Models\Performance;
+
+/**
+ * Observer for Performance.
+ *
+ * Increments the optimistic-lock `version` column by 1 on every UPDATE
+ * (not on INSERT). RFC v0.2 D14 — `MoveTimetablePerformanceRequest`
+ * (Session 2) compares the client-supplied version against the stored
+ * value to detect concurrent edits.
+ */
+final class PerformanceObserver
+{
+    public function saving(Performance $performance): void
+    {
+        if ($performance->exists && $performance->isDirty()) {
+            $performance->version = (int) $performance->version + 1;
+        }
+    }
+}