feat: registration form fields, section preferences, tag sync & schema updates

Implement EAV system for dynamic event-specific registration fields
with organisation-level templates, person section preferences with
priority ranking, and TagSyncService for deferred tag_picker sync.

New tables: registration_field_templates, registration_form_fields,
person_field_values, person_section_preferences.
New columns: persons.remarks, events.registration_show_section_preferences,
events.registration_show_availability.

58 tests, 126 assertions — all 432 tests pass (zero regressions).

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

api/app/Models/Person.php

@@ -36,6 +36,7 @@ final class Person extends Model
         'status',
         'is_blacklisted',
         'admin_notes',
+        'remarks',
         'custom_fields',
     ];
 
@@ -94,6 +95,16 @@ final class Person extends Model
         return $this->hasMany(VolunteerAvailability::class);
     }
 
+    public function fieldValues(): HasMany
+    {
+        return $this->hasMany(PersonFieldValue::class);
+    }
+
+    public function sectionPreferences(): HasMany
+    {
+        return $this->hasMany(PersonSectionPreference::class);
+    }
+
     public function identityMatches(): HasMany
     {
         return $this->hasMany(PersonIdentityMatch::class);