-- Which alert source an alert last arrived on.
--
-- Team -> Sources shows when each source last posted, which integrations
-- already knew (last_used_at, stamped on every webhook). What it could not say
-- was what a source delivered: an alert never recorded the key it came in on, so
-- "prod alertmanager" and "staging alertmanager" were indistinguishable once
-- inside. This column is that link, and lets the page show each source's last
-- alert and how many alerts it has kept fresh over the past day.
--
-- Last sender wins: every accepted payload restamps it, the way it advances
-- received_at. Two sources posting the same fingerprint into one team is
-- already one alert, and it is attributed to whichever spoke last.
--
-- Nullable, and not backfilled. Alerts that arrived before this migration have
-- no source, and NULL says so honestly rather than guessing. It heals by itself:
-- Alertmanager re-sends every alert each repeat_interval, and each re-send is an
-- accepted payload. Deleting a source keeps its alerts, unattributed.
ALTER TABLE alerts ADD COLUMN integration_id BIGINT REFERENCES integrations(id) ON DELETE SET NULL;

CREATE INDEX alerts_integration_idx ON alerts (integration_id, received_at)
    WHERE integration_id IS NOT NULL;