refactor(form-builder): drop custom purpose escape from schemas

Reduces the FormPurpose vocabulary from 22 variants + a `custom` escape
to the seven v1.0 purposes registered in the new PurposeRegistry.

- Purge migration deletes any form_schemas row whose `purpose` is not
  in the v1.0 set (cascades through form_fields, form_submissions,
  form_values, form_value_options, form_schema_sections,
  form_submission_section_statuses, form_submission_delegations,
  form_schema_webhooks, form_webhook_deliveries via existing FK).
- Drop migration removes the `custom_purpose_slug` column + its index.
- Both migrations declare their `down()` as a hard failure — we do not
  support reversing a purge (pre-launch, no production data).
- `FormPurpose` enum slims to the seven cases; the legacy helpers
  (defaultSubmissionMode / defaultSubjectType / allowsPublicAccess)
  now delegate to PurposeRegistry so callers keep working.
- FormSchema fillable / FormSchemaResource / StoreFormSchemaRequest /
  UpdateFormSchemaRequest / FormSchemaFactory drop every reference to
  `custom_purpose_slug` and the `custom` purpose.
- VerifyFormsDataIntegrity drops the custom-slug mismatch check and
  sources the subject-type allow-list from PurposeRegistry.

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

api/database/factories/FormBuilder/FormSchemaFactory.php

@@ -6,7 +6,6 @@ namespace Database\Factories\FormBuilder;
 
 use App\Enums\FormBuilder\FormPurpose;
 use App\Enums\FormBuilder\FormSchemaSnapshotMode;
-use App\Enums\FormBuilder\FormSubmissionMode;
 use App\Models\FormBuilder\FormSchema;
 use App\Models\Organisation;
 use Illuminate\Database\Eloquent\Factories\Factory;
@@ -22,9 +21,9 @@ final class FormSchemaFactory extends Factory
     {
         $purpose = fake()->randomElement([
             FormPurpose::EVENT_REGISTRATION,
-            FormPurpose::FEEDBACK,
             FormPurpose::INCIDENT_REPORT,
             FormPurpose::USER_PROFILE,
+            FormPurpose::POST_EVENT_EVALUATION,
         ]);
         $name = 'Formulier '.fake('nl_NL')->words(2, true);
 
@@ -35,7 +34,6 @@ final class FormSchemaFactory extends Factory
             'name' => $name,
             'slug' => Str::slug($name).'-'.Str::lower(Str::random(4)),
             'purpose' => $purpose,
-            'custom_purpose_slug' => null,
             'description' => fake('nl_NL')->sentence(),
             'is_published' => false,
             'submission_mode' => $purpose->defaultSubmissionMode(),
@@ -48,15 +46,6 @@ final class FormSchemaFactory extends Factory
         ];
     }
 
-    public function custom(string $slug): static
-    {
-        return $this->state(fn () => [
-            'purpose' => FormPurpose::CUSTOM,
-            'submission_mode' => FormSubmissionMode::SINGLE,
-            'custom_purpose_slug' => $slug,
-        ]);
-    }
-
     public function published(): static
     {
         return $this->state(fn () => ['is_published' => true]);
@@ -67,7 +56,6 @@ final class FormSchemaFactory extends Factory
         return $this->state(fn () => [
             'purpose' => $purpose,
             'submission_mode' => $purpose->defaultSubmissionMode(),
-            'custom_purpose_slug' => $purpose === FormPurpose::CUSTOM ? 'custom-'.Str::lower(Str::random(6)) : null,
         ]);
     }
 }