fix(form-builder): restore FK on form_schemas.default_crowd_type_id (WS-6)

The original session 2.5 migration had to omit this FK due to an
SQLite-only "rebuild on FK add" cascade-delete quirk. Now that the
test infrastructure has moved to MySQL (Task 1 of this session), the
quirk does not apply and the FK is restored to match every other FK
in this table.

Changes:
- New migration `2026_04_28_100000_restore_default_crowd_type_id_foreign_key`
  adds a FOREIGN KEY (default_crowd_type_id) REFERENCES crowd_types(id)
  ON DELETE SET NULL. Deleting a CrowdType nulls the column on dependent
  schemas instead of cascading the schema delete.
- Original migration's comment block rewritten — the SQLite-quirk
  rationale was demonstrably misleading; replaced with a forward-looking
  pointer to the FK-restore migration.
- PersonProvisioner::resolveCrowdTypeId() docblock updated: the runtime
  failsafe is now defense in depth alongside the DB-level FK + publish
  guard, not the sole load-bearing check.

New test (`DefaultCrowdTypeForeignKeyTest`) exercises both the
ON-DELETE-SET-NULL cascade and the existence of the FK in
information_schema.REFERENTIAL_CONSTRAINTS — the second assertion would
have been impossible on SQLite, which is exactly the point.

Migration step counts in 5 backfill tests bumped +1 because the FK-
restore migration sits at the top of the migration stack:
  - FormFieldBindingMigrationTest:           17→18, 15→16
  - ConditionalLogicBackfillTest:             6→7
  - FormFieldConfigBackfillAndDropTest:      12→13
  - FormFieldOptionsBackfillTest:             2→3
  - FormFieldValidationRuleBackfillTest:     15→16

All 1388 tests pass on MySQL (1386 prior + 2 new FK tests). Larastan
baseline unchanged.

Refs: RFC-WS-6.md v1.1 §3 Q9 addendum, WS-6 session 2.5 deviation #1

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

api/app/FormBuilder/Bindings/PersonProvisioner.php

@@ -156,10 +156,11 @@ final readonly class PersonProvisioner
      * declares its target CrowdType explicitly via `default_crowd_type_id`.
      *
      * RFC-WS-6 v1.1 §3 Q9 addendum (was: silent oldest() fallback in
-     * session 2). The RequiresDefaultCrowdType publish guard prevents
-     * misconfigured event_registration schemas from publishing; this
-     * runtime throw is a failsafe for live-table edits between publish
-     * and apply.
+     * session 2). Defense in depth: the FK on form_schemas (added in
+     * session 2.6 after the SQLite → MySQL test switch) gives DB-level
+     * referential integrity; the RequiresDefaultCrowdType publish guard
+     * blocks publish when null; this runtime throw is the failsafe for
+     * live-table edits between publish and apply.
      *
      * @throws PersonProvisioningException
      */