store: migration 0013 — alerts.last_seen_at

internal/store/migrations/0013_alerts_last_seen.sql

@@ -0,0 +1,16 @@
+-- 0013_alerts_last_seen.sql
+--
+-- Add alerts.last_seen_at to support open-alert dedup with
+-- recurrence-tracking. The engine bumps this column on every tick
+-- where a rule still matches an existing open alert, so the UI can
+-- render "still happening · Ns ago" without sending a fresh
+-- notification.
+--
+-- Column-level ALTER per CLAUDE.md (no rebuild — alerts has inbound
+-- FK from acknowledged_by → users; rebuild would risk cascade).
+-- Backfill last_seen_at = created_at for any pre-existing rows so
+-- the column is non-null in practice (stays nullable in the schema
+-- for forwards-compat with rows that haven't been touched yet).
+
+ALTER TABLE alerts ADD COLUMN last_seen_at TEXT;
+UPDATE alerts SET last_seen_at = created_at WHERE last_seen_at IS NULL;