fix(timetable): config-flag observer + cleaner idempotency_key

OrganisationObserver was gated on app()->runningUnitTests() — replaced
with config('artist_advance.bootstrap_on_org_create') (default true,
phpunit.xml overrides to false). Behaviour identical, but the seam is
explicit and removable. Tracked for full convergence by new BACKLOG
entry TECH-OBSERVER-TEST-CONVERGENCE — productiegedrag = testgedrag,
geen branching, na test-cleanup.

idempotency_key for the engagement-scoped draft simplified from
'aa-' + sha1(engagement_id)[0:27] to 'aa:' + engagement_id (29 chars,
fits varchar(30)). Same uniqueness guarantee, recognisable shape.

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

api/app/Observers/OrganisationObserver.php

@@ -18,15 +18,18 @@ use App\Models\Organisation;
  * The default seeder is idempotent — if the org already owns an
  * artist_advance schema, the call is a no-op. Safe to re-run.
  *
- * Skipped during automated tests so existing FormSchema-counting
- * tests aren't perturbed; tests that need the auto-seed call
- * `ArtistAdvanceDefault::seedFor()` explicitly.
+ * Gated by `config('artist_advance.bootstrap_on_org_create')`. The
+ * config defaults to true (production behaviour); phpunit.xml flips
+ * it to false so existing FormSchema-counting tests aren't perturbed.
+ * Tests that need the auto-seed call `ArtistAdvanceDefault::seedFor()`
+ * explicitly. Tracked for removal by BACKLOG entry
+ * `TECH-OBSERVER-TEST-CONVERGENCE`.
  */
 final class OrganisationObserver
 {
     public function created(Organisation $organisation): void
     {
-        if (app()->runningUnitTests()) {
+        if (! (bool) config('artist_advance.bootstrap_on_org_create', true)) {
             return;
         }