CREATE TABLE notifications (
    id              INTEGER PRIMARY KEY,
    recipient       TEXT NOT NULL,
    subject         TEXT NOT NULL,
    body            TEXT NOT NULL,
    attempts        INTEGER NOT NULL DEFAULT 0,
    next_attempt_at TEXT,
    sent_at         TEXT,
    failed_at       TEXT,
    last_error      TEXT,
    created_at      TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ','now'))
);
CREATE INDEX notifications_due ON notifications(next_attempt_at)
    WHERE sent_at IS NULL AND failed_at IS NULL;