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/config/artist_advance.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+return [
+    /*
+    |---------------------------------------------------------------------
+    | Bootstrap on organisation create
+    |---------------------------------------------------------------------
+    |
+    | When true (production default), OrganisationObserver provisions a
+    | default artist_advance FormSchema for every newly-created
+    | Organisation per RFC-TIMETABLE v0.2 D15. When false, the observer
+    | no-ops and tests opt in to the seeder explicitly via
+    | `ArtistAdvanceDefault::seedFor()`.
+    |
+    | Tracked for removal by BACKLOG: TECH-OBSERVER-TEST-CONVERGENCE —
+    | once existing FormSchema-counting tests are updated to expect the
+    | auto-bootstrapped schema, this flag goes away (productiegedrag =
+    | testgedrag, geen branching).
+    |
+    */
+
+    'bootstrap_on_org_create' => env('ARTIST_ADVANCE_BOOTSTRAP_ON_ORG_CREATE', true),
+];