CREATE TABLE users (
    id SERIAL PRIMARY KEY,
    username VARCHAR(50) UNIQUE NOT NULL,
    password_hash VARCHAR(255) NOT NULL
);

CREATE TABLE messages (
    id SERIAL PRIMARY KEY,
    sender_id INT REFERENCES users(id),
    receiver_id INT REFERENCES users(id),
    content TEXT NOT NULL,
    sent_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
);