CREATE TABLE IF NOT EXISTS messages (
    id INT AUTO_INCREMENT PRIMARY KEY,
    sender_id INT NOT NULL,
    sender_type ENUM('coach', 'client') NOT NULL,
    receiver_id INT NOT NULL,
    receiver_type ENUM('coach', 'client') NOT NULL,
    message TEXT NOT NULL,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    is_read BOOLEAN DEFAULT FALSE
);