CREATE TABLE IF NOT EXISTS fcm_tokens (
    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
    token TEXT NOT NULL,
    platform TEXT NOT NULL CHECK (platform IN ('web', 'android', 'ios')),
    last_seen_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
    created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
    
    UNIQUE(user_id, token)
);

CREATE INDEX IF NOT EXISTS idx_fcm_tokens_last_seen_at ON fcm_tokens(last_seen_at);