feat: registration section preferences with show_in_registration filtering and deduplication

Add show_in_registration and registration_description columns to festival_sections.
Registration form now shows deduplicated sections by name (across sub-events),
filtered by show_in_registration=true, grouped by category with card-based UI.
Section preferences use section_name instead of section_id.
Add GET/PUT registration-settings endpoints for festival-level bulk management.

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

api/app/Models/FestivalSection.php

@@ -33,6 +33,8 @@ final class FestivalSection extends Model
         'crew_accreditation_level',
         'public_form_accreditation_level',
         'timed_accreditations',
+        'show_in_registration',
+        'registration_description',
     ];
 
     protected function casts(): array
@@ -43,6 +45,7 @@ final class FestivalSection extends Model
             'added_to_timeline' => 'boolean',
             'responder_self_checkin' => 'boolean',
             'timed_accreditations' => 'boolean',
+            'show_in_registration' => 'boolean',
         ];
     }