feat(form-builder): add apply_status columns and action-failures table (WS-6)

- form_submissions: apply_status (nullable, NO default for legacy rows
  per RFC O1), apply_completed_at, indexed on (form_schema_id, apply_status)
  and (organisation_id, apply_status)
- form_submission_action_failures: ULID PK, FK to submission + binding,
  resolve/dismiss state separated (RFC V2), retention via parent
  cascade-delete
- Migration rehearsal test added (invokes down() directly because the new
  migrations land between WS-5a and WS-5b chronologically, not at the tail
  of the migration list)

Three pre-existing WS-5 backfill tests also bump their --step rollback
counts by +2 (FormFieldBindingMigrationTest, FormFieldConfigBackfillAndDropTest,
FormFieldValidationRuleBackfillTest) to account for the two new migrations
sitting in the chronological middle of the WS-5 stack — required to keep
those tests' pre-WS-5b rollback target reachable.

SCHEMA.md updated to v2.3.
Refs: RFC-WS-6.md §3 (Q4, Q5), §4 (V2), §5 (O1)

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

api/tests/Feature/FormBuilder/Configs/FormFieldConfigBackfillAndDropTest.php

@@ -27,10 +27,10 @@ final class FormFieldConfigBackfillAndDropTest extends TestCase
 
     public function test_backfill_translates_tag_categories_and_storage_disk(): void
     {
-        // Roll back 2 WS-5c migrations + 5 WS-5b migrations = 7, to get the
-        // pre-WS-5b state where the JSON column still exists on form_fields
-        // / form_field_library.
-        $this->artisan('migrate:rollback', ['--step' => 9])->assertSuccessful();
+        // Roll back 4 WS-5c migrations + 2 WS-6 migrations + 5 WS-5b
+        // migrations = 11, to get the pre-WS-5b state where the JSON column
+        // still exists on form_fields / form_field_library.
+        $this->artisan('migrate:rollback', ['--step' => 11])->assertSuccessful();
         $this->assertTrue(Schema::hasColumn('form_fields', 'validation_rules'));
 
         $fieldId = $this->seedField([