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/Services/FormBuilder/FormValueService.php

@@ -340,9 +340,8 @@ final class FormValueService
         }
 
         if ($subjectType === $entity) {
-            $map = config('form_subjects');
-            $model = $map[$entity]['model'] ?? null;
-            if ($model === null) {
+            $model = \Illuminate\Database\Eloquent\Relations\Relation::getMorphedModel($entity);
+            if ($model === null || ! class_exists($model)) {
                 return null;
             }