feat(forms): add PHP enums for form builder

9 backed string enums covering purpose, field type, submission status/mode/review,
field width, value storage hint, snapshot mode, webhook delivery status.
FormPurpose/FormFieldType include helper methods per ARCH §3/§5. All with
declare(strict_types=1) and values() helpers for validation rules.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/app/Enums/FormBuilder/FormFieldDisplayWidth.php

@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Enums\FormBuilder;
+
+enum FormFieldDisplayWidth: string
+{
+    case HALF = 'half';
+    case FULL = 'full';
+
+    /**
+     * @return array<int, string>
+     */
+    public static function values(): array
+    {
+        return array_map(fn (self $case): string => $case->value, self::cases());
+    }
+}