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/phpunit.xml

@@ -35,6 +35,7 @@
         <env name="PULSE_ENABLED" value="false"/>
         <env name="TELESCOPE_ENABLED" value="false"/>
         <env name="NIGHTWATCH_ENABLED" value="false"/>
+        <env name="ARTIST_ADVANCE_BOOTSTRAP_ON_ORG_CREATE" value="false"/>
         <ini name="memory_limit" value="512M"/>
     </php>
 </phpunit>