CREATE TABLE notifications (
  id           INTEGER PRIMARY KEY AUTOINCREMENT,
  user_id      INTEGER NOT NULL,
  actor_id     INTEGER,
  actor_name   TEXT,
  action       TEXT NOT NULL,
  entity_type  TEXT NOT NULL,
  entity_id    INTEGER NOT NULL,
  entity_title TEXT NOT NULL,
  entity_slug  TEXT,
  read_at      DATETIME,
  created_at   DATETIME DEFAULT CURRENT_TIMESTAMP,
  FOREIGN KEY (user_id)  REFERENCES users(id) ON DELETE CASCADE,
  FOREIGN KEY (actor_id) REFERENCES users(id) ON DELETE SET NULL
);
CREATE INDEX notifications_user_unread ON notifications (user_id, read_at);