feat(attachments): add AttachmentType enum and update Attachment model to use it

database/migrations/2026_06_25_000500_create_attachments_table.php

@@ -18,7 +18,7 @@ return new class extends Migration
             $table->uuid('key')->unique();
             $table->string('path');
             $table->string('filename');
-            $table->string('type')->nullable();
+            $table->string('type');
             $table->string('mime_type');
             $table->string('extension', 20)->nullable();
             $table->unsignedBigInteger('size')->default(0);