Remember when a Site called a work completed (#169)

Add series.site_completed_at (epoch-ms, default 0): the durable answer
#168's predicates compute. checkOne writes it after the successful read
on the zero/non-zero transition only, so a Series still completed keeps
its original stamp (the age #170 prints is 'since the Site first said
so'), one that stopped is zeroed, one that became completed is stamped.
The due query projects the column, so the transition is a field
comparison on the snapshot checkOne already holds.

Refused, unreachable and errored reads reach nothing (AC4 is placement,
not a guard); the write sits before the no-chapter and unchanged-number
returns, so a completed page whose chapter did not change still writes.
Store failure logs and carries on — the outcome word never changes.

Series-level like Latest Chapter: a bookmark's updated_at is never
touched. #170 is the surface; nothing reads the column yet.

backend/internal/store/store.go

@@ -84,6 +84,10 @@ type Series struct {
 	LatestChapter    string
 	LatestChapterNum *float64 // nil until first captured
 	LatestCheckedAt  int64    // unix ms; see MarkLatestChecked
+	// SiteCompletedAt is when the last successful Poll read saw the Site's
+	// own completed value, zero meaning it did not (issue #168). A poller
+	// fact, not reading progress: Upsert never touches it.
+	SiteCompletedAt int64
 	// LatestRaisedBy is the Reader whose Sighting last raised LatestChapter,
 	// and nil when the stored value is a Poll's own finding. It is what lets a
 	// Poll that contradicts the value downwards name a Reader instead of
@@ -240,7 +244,8 @@ const bookmarkColumns = `b.site, b.series_id, s.title, s.series_url, s.cover_add
 // due query. latest_checked_at lives only on series — see MarkLatestChecked
 // for why it stays off every client-visible write.
 const seriesColumns = `s.site, s.series_id, s.title, s.series_url, s.cover, s.cover_address,
-	s.kind, s.latest_chapter, s.latest_chapter_num, s.latest_checked_at, s.latest_raised_by`
+	s.kind, s.latest_chapter, s.latest_chapter_num, s.latest_checked_at, s.latest_raised_by,
+	s.site_completed_at`
 
 const lanePassColumns = `p.site, p.ran_at, p.skip, p.due, p.checked, p.gap_ms, p.clamped,
 	p.refused, p.unreachable, p.no_chapter, p.unfetchable, p.errors, p.not_found,
@@ -646,7 +651,7 @@ func scanSeries(scan func(...any) error) (Series, error) {
 	if err := scan(
 		&sr.Site, &sr.SeriesID, &sr.Title, &sr.SeriesURL, &sr.Cover, &sr.CoverAddress,
 		&sr.Kind, &sr.LatestChapter, &latestChapterNum, &sr.LatestCheckedAt, &latestRaisedBy,
-		&sr.Forced, &sr.readerCount,
+		&sr.SiteCompletedAt, &sr.Forced, &sr.readerCount,
 	); err != nil {
 		return Series{}, err
 	}
@@ -1356,7 +1361,7 @@ func (s *Store) DueForLatestCheck(site string, cutoffMs, ceilingMs int64) ([]Ser
 		  AND (s.finished_at = 0 OR s.force_poll_at > s.latest_checked_at)
 		GROUP BY s.site, s.series_id, s.title, s.series_url, s.cover,
 		         s.kind, s.latest_chapter, s.latest_chapter_num, s.latest_checked_at,
-		         s.force_poll_at, s.finished_at
+		         s.site_completed_at, s.force_poll_at, s.finished_at
 		HAVING (COUNT(*) > 1
 		        OR s.latest_sighted_at <= $2::bigint
 		        OR s.latest_checked_at <= $3::bigint
@@ -1491,6 +1496,22 @@ func (s *Store) SetLatestChapter(site, seriesID, label string, num float64) erro
 	return nil
 }
 
+// SetSiteCompletedAt stores when the last successful read saw the Site's
+// completed value, zero meaning it did not. Series-level, like the Latest
+// Chapter: it is a fact about the work, not about one Reader's bookmark, and
+// the bookmark's updated_at is never touched — this is not reading progress
+// and must not reorder any Reader's list, the same rule SetLatestChapter's
+// comment states. Touching a missing series is not an error: the row may
+// have been orphaned, and the caller's read decides what exists.
+func (s *Store) SetSiteCompletedAt(site, seriesID string, at int64) error {
+	if _, err := s.db.Exec(
+		`UPDATE series SET site_completed_at = $1 WHERE site = $2 AND series_id = $3`,
+		at, site, seriesID); err != nil {
+		return fmt.Errorf("set site completed %s:%s: %w", site, seriesID, err)
+	}
+	return nil
+}
+
 // SetSeriesURL stores the owner's repair for a Series' source address
 // (issue #151): the one write that lifts the write-once rule documented on
 // Series.SeriesURL. It is a store, not a verification — the caller has