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/Controllers/Api/V1/FormBuilder/FormSubmissionController.php

@@ -14,6 +14,7 @@ use App\Models\FormBuilder\FormSubmission;
 use App\Models\Organisation;
 use App\Services\FormBuilder\FormSubmissionService;
 use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\Relation;
 use Illuminate\Http\JsonResponse;
 use Illuminate\Http\Resources\Json\AnonymousResourceCollection;
 use Illuminate\Support\Facades\Gate;
@@ -103,9 +104,8 @@ final class FormSubmissionController extends Controller
         if ($type === '' || $id === '') {
             return null;
         }
-        $map = (array) config('form_subjects');
-        $class = $map[$type]['model'] ?? null;
-        if ($class === null) {
+        $class = Relation::getMorphedModel($type);
+        if ($class === null || ! class_exists($class)) {
             return null;
         }