fix(timetable): null-on-delete advance_submissions per RFC §5.4 retention

advance_submissions.advance_section_id FK changed from cascadeOnDelete
to nullOnDelete; column made nullable. Aligns implementation with
RFC v0.2 §5.4 audit-immutability ("submissions remain for retention
compliance") — when ArtistEngagementObserver::deleted hard-deletes a
section, its submissions persist as orphans rather than disappearing.

Migration edited in place (branch unpushed, dev-only). Observer
docblock + test assertion updated to match. Removed pre-existing
follow-up comment that documented the deviation.

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

api/app/Observers/ArtistEngagementObserver.php

@@ -15,10 +15,10 @@ use App\Models\Scopes\OrganisationScope;
  * (RFC v0.2 D10) and asserts artist.organisation_id === event.organisation_id.
  *
  * On `deleted` (soft delete): cascade soft-delete to child Performance
- * rows; hard-delete child AdvanceSection rows (RFC §5.4 — sections have
- * no soft delete). Child AdvanceSubmission rows are immutable audit
- * records and remain attached to their (now-deleted) section reference
- * by FK — this is the audit-retention compliance path.
+ * rows; hard-delete child AdvanceSection rows (per RFC §5.4 — sections
+ * have no soft delete). AdvanceSubmission rows remain in the table with
+ * `advance_section_id = NULL` (FK is `nullOnDelete`) per RFC §5.4
+ * audit-immutability — orphaned but preserved for retention compliance.
  */
 final class ArtistEngagementObserver
 {