Record poll failures as rows: the row is the failure state (#165)

backend/internal/store/store.go

@@ -1297,6 +1297,31 @@ func (s *Store) LaneGates(site string) (pausedUntil, refuseUntil int64, err erro
 	}
 	return pausedUntil, refuseUntil, nil
 }
+// RecordSeriesFailure upserts one Series' failure row: the outcome word is
+// updated when it changes, failing_since is never overwritten, and an
+// unchanged word writes nothing. One statement, so the identical-word no-op
+// and the keep-the-stamp rule are the same guarantee: the SET arm fires only
+// when the word differs, and failing_since is simply absent from it
+// (ADR-0016).
+func (s *Store) RecordSeriesFailure(site, seriesID, outcome string, now int64) error {
+	if _, err := s.db.Exec(`
+		INSERT INTO poll_failures (site, series_id, outcome, failing_since) VALUES ($1, $2, $3, $4)
+		ON CONFLICT (site, series_id) DO UPDATE SET outcome = EXCLUDED.outcome
+		WHERE poll_failures.outcome <> EXCLUDED.outcome`, site, seriesID, outcome, now); err != nil {
+		return fmt.Errorf("record series failure %s:%s: %w", site, seriesID, err)
+	}
+	return nil
+}
+
+// ClearSeriesFailure deletes one Series' failure row. A row that is not there
+// is not an error.
+func (s *Store) ClearSeriesFailure(site, seriesID string) error {
+	if _, err := s.db.Exec(
+		`DELETE FROM poll_failures WHERE site = $1 AND series_id = $2`, site, seriesID); err != nil {
+		return fmt.Errorf("clear series failure %s:%s: %w", site, seriesID, err)
+	}
+	return nil
+}
 
 // DueForLatestCheck returns one Site's series whose server-side
 // latest-chapter check has aged past cutoffMs, ordered by how many bookmarks