CREATE TABLE audit_outbox (
  event_id TEXT PRIMARY KEY NOT NULL,
  payload_json TEXT NOT NULL CHECK(length(payload_json) <= 16384),
  attempts INTEGER NOT NULL DEFAULT 0 CHECK(attempts >= 0),
  next_attempt_ms INTEGER NOT NULL DEFAULT 0,
  created_at TEXT NOT NULL,
  last_error TEXT
);
CREATE INDEX idx_audit_outbox_due ON audit_outbox(next_attempt_ms, created_at);