add add user method

migrations/20260628175120_create_users_table.sql

@@ -4,7 +4,7 @@ CREATE TABLE IF NOT EXISTS users (
     username VARCHAR(50) NOT NULL UNIQUE,
     email VARCHAR(100) NOT NULL UNIQUE,
     password_hash VARCHAR(255) NOT NULL,
-    role VARCHAR(20) DEFAULT 'user' CHECK (role IN ('admin', 'author', 'organizer', 'user')),
+    role VARCHAR(20) CHECK (role IN ('admin', 'author', 'organizer', 'user')),
     is_active BOOLEAN DEFAULT TRUE,
     created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
 );