feat(api): extend registration endpoints with dynamic fields and section preferences

- PublicRegistrationData now returns registration_fields (portal-visible only),
  form toggles, and available_tags for tag_picker fields
- Volunteer registration accepts field_values and section_preferences with
  festival_section_id, processed via existing services
- PortalMe eager-loads fieldValues and sectionPreferences
- Section preferences now use the proper relational table instead of custom_fields JSON

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/Http/Requests/Api/V1/VolunteerRegistrationRequest.php

@@ -46,12 +46,14 @@ final class VolunteerRegistrationRequest extends FormRequest
             'motivation_other' => ['nullable', 'string', 'max:500'],
 
             'section_preferences' => ['nullable', 'array', 'max:5'],
-            'section_preferences.*.section_name' => ['required', 'string', 'max:255'],
+            'section_preferences.*.festival_section_id' => ['required', 'ulid'],
             'section_preferences.*.priority' => ['required', 'integer', 'min:1', 'max:5'],
 
             'availabilities' => ['nullable', 'array'],
             'availabilities.*.time_slot_id' => ['required', 'ulid', 'exists:time_slots,id'],
             'availabilities.*.preference_level' => ['nullable', 'integer', 'min:1', 'max:5'],
+
+            'field_values' => ['nullable', 'array'],
         ];
     }
 }