refactor(form-builder): consolidate subject-type allow-list into purpose registry

Q6 of ARCH-CONSOLIDATION-ADDENDUM-2026-04-24: the allowed
`form_submissions.subject_type` values are now derived from
`PurposeRegistry::allSubjectTypes()` instead of the parallel
`config/form_subjects.php` file.

- CreateFormSubmissionRequest validates `subject_type` against the
  registry via constructor-injected PurposeRegistry.
- FormSubmissionController and FormValueService resolve the subject
  FQCN through `Relation::getMorphedModel()` — the morph-map is the
  single source of truth for alias → model mapping.
- `config/form_subjects.php` is deleted. `MorphMapAlignmentTest` keeps
  the registry and morph-map aligned going forward.

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

api/app/Http/Requests/Api/V1/FormBuilder/CreateFormSubmissionRequest.php

@@ -4,11 +4,18 @@ declare(strict_types=1);
 
 namespace App\Http\Requests\Api\V1\FormBuilder;
 
+use App\FormBuilder\Purposes\PurposeRegistry;
 use Illuminate\Foundation\Http\FormRequest;
 use Illuminate\Validation\Rule;
 
 final class CreateFormSubmissionRequest extends FormRequest
 {
+    public function __construct(
+        private readonly PurposeRegistry $purposeRegistry,
+    ) {
+        parent::__construct();
+    }
+
     public function authorize(): bool
     {
         return true;
@@ -19,10 +26,8 @@ final class CreateFormSubmissionRequest extends FormRequest
      */
     public function rules(): array
     {
-        $allowedSubjects = array_keys((array) config('form_subjects', []));
-
         return [
-            'subject_type' => ['nullable', Rule::in($allowedSubjects)],
+            'subject_type' => ['nullable', Rule::in($this->purposeRegistry->allSubjectTypes())],
             'subject_id' => ['nullable', 'string', 'max:30', 'required_with:subject_type'],
             'idempotency_key' => ['nullable', 'string', 'max:30'],
             'is_test' => ['boolean'],