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/Organisation.php

@@ -62,4 +62,9 @@ final class Organisation extends Model
     {
         return $this->hasMany(PersonTag::class);
     }
+
+    public function registrationFieldTemplates(): HasMany
+    {
+        return $this->hasMany(RegistrationFieldTemplate::class);
+    }
 }