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/config/form_subjects.php

@@ -1,52 +0,0 @@
-<?php
-
-declare(strict_types=1);
-
-/*
-|--------------------------------------------------------------------------
-| Subject Type Registry
-|--------------------------------------------------------------------------
-|
-| Authoritative list of subject_type values permitted on form_submissions.
-| Must match morph-map keys registered in AppServiceProvider.
-|
-| 'permission_check' format: '<PolicyClass>@<method>' — invoked to authorise
-| access to a submission for this subject. Omit when policy doesn't exist yet.
-|
-*/
-
-return [
-
-    'person' => [
-        'model' => \App\Models\Person::class,
-        'display_attribute' => 'name',
-        'permission_check' => \App\Policies\PersonPolicy::class.'@view',
-    ],
-
-    'user' => [
-        'model' => \App\Models\User::class,
-        'display_attribute' => 'name',
-        // TODO: add permission_check when UserPolicy is built (S2)
-    ],
-
-    'company' => [
-        'model' => \App\Models\Company::class,
-        'display_attribute' => 'name',
-        'permission_check' => \App\Policies\CompanyPolicy::class.'@view',
-    ],
-
-    'organisation' => [
-        'model' => \App\Models\Organisation::class,
-        'display_attribute' => 'name',
-        'permission_check' => \App\Policies\OrganisationPolicy::class.'@view',
-    ],
-
-    'event' => [
-        'model' => \App\Models\Event::class,
-        'display_attribute' => 'name',
-        'permission_check' => \App\Policies\EventPolicy::class.'@view',
-    ],
-
-    // 'artist' entry added when artist module lands
-
-];