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/tests/Feature/FormBuilder/ValidationRules/FormFieldValidationRuleBackfillTest.php

@@ -53,7 +53,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // validation-rules-backfill + create-validation-rules) = 14.
         // Brings us to the pre-WS-5b state: validation_rules JSON column
         // present, no relational tables for WS-5b/c/d.
-        $this->artisan('migrate:rollback', ['--step' => 15])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 16])->assertSuccessful();
         $this->assertFalse(Schema::hasTable('form_field_validation_rules'));
         $this->assertTrue(Schema::hasColumn('form_fields', 'validation_rules'));
 
@@ -114,7 +114,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // (validation_rules JSON column present; no relational tables for
         // WS-5b). Step count: drop-cols + configs-backfill + create-configs
         // + validation-rules-backfill + create-validation-rules = 5.
-        $this->artisan('migrate:rollback', ['--step' => 15])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 16])->assertSuccessful();
 
         $fieldId = $this->seedFieldWithJson([
             'field_type' => 'TAG_PICKER',
@@ -138,7 +138,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // (validation_rules JSON column present; no relational tables for
         // WS-5b). Step count: drop-cols + configs-backfill + create-configs
         // + validation-rules-backfill + create-validation-rules = 5.
-        $this->artisan('migrate:rollback', ['--step' => 15])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 16])->assertSuccessful();
 
         $fieldId = $this->seedFieldWithJson([
             'field_type' => 'TEXT',
@@ -165,7 +165,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // (validation_rules JSON column present; no relational tables for
         // WS-5b). Step count: drop-cols + configs-backfill + create-configs
         // + validation-rules-backfill + create-validation-rules = 5.
-        $this->artisan('migrate:rollback', ['--step' => 15])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 16])->assertSuccessful();
 
         $this->seedFieldWithJson([
             'field_type' => 'TEXT',
@@ -182,7 +182,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // (validation_rules JSON column present; no relational tables for
         // WS-5b). Step count: drop-cols + configs-backfill + create-configs
         // + validation-rules-backfill + create-validation-rules = 5.
-        $this->artisan('migrate:rollback', ['--step' => 15])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 16])->assertSuccessful();
 
         $this->seedFieldWithJson([
             'field_type' => 'BOOLEAN',
@@ -201,7 +201,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
         // full-back-then-full-forward cycle — rolling back all WS-5b
         // migrations restores the pre-WS-5b state (columns present on
         // source tables; validation rules relational table gone).
-        $this->artisan('migrate:rollback', ['--step' => 15])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 16])->assertSuccessful();
         [$numberId] = $this->seedFields();
 
         $this->artisan('migrate')->assertSuccessful();
@@ -216,7 +216,7 @@ final class FormFieldValidationRuleBackfillTest extends TestCase
 
         // Roll back WS-5b fully → column reappears and carries canonical JSON
         // reconstructed from the relational rows.
-        $this->artisan('migrate:rollback', ['--step' => 15])->assertSuccessful();
+        $this->artisan('migrate:rollback', ['--step' => 16])->assertSuccessful();
         $this->assertTrue(Schema::hasColumn('form_fields', 'validation_rules'));
 
         $field = DB::table('form_fields')->where('id', $numberId)->first();