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/PersonFieldValue.php

@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+final class PersonFieldValue extends Model
+{
+    public $timestamps = false;
+
+    protected $fillable = [
+        'person_id',
+        'registration_form_field_id',
+        'value',
+        'selected_options',
+    ];
+
+    protected function casts(): array
+    {
+        return [
+            'selected_options' => 'array',
+        ];
+    }
+
+    public function person(): BelongsTo
+    {
+        return $this->belongsTo(Person::class);
+    }
+
+    public function registrationFormField(): BelongsTo
+    {
+        return $this->belongsTo(RegistrationFormField::class);
+    }
+}