feat(attachments): add 'key' field to Attachment model and migration, generate UUID on creation

database/migrations/2026_06_25_000500_create_attachments_table.php

@@ -15,6 +15,7 @@ return new class extends Migration
             $table->id();
             $table->string('attachable_type');
             $table->unsignedBigInteger('attachable_id');
+            $table->uuid('key')->unique();
             $table->string('path');
             $table->string('filename');
             $table->string('type')->nullable();