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/app/Models/FormBuilder/FormSchema.php

@@ -42,7 +42,6 @@ final class FormSchema extends Model
         'name',
         'slug',
         'purpose',
-        'custom_purpose_slug',
         'description',
         'is_published',
         'submission_mode',