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/latest/poller.go

@@ -678,6 +678,25 @@ func (p *Poller) checkOne(ctx context.Context, sr store.Series) (outcome readOut
 	} else {
 		p.fillBlankCover(ctx, sr, facts.Cover)
 	}
+
+	// Learned from the successful read: the write sits after the error switch
+	// (a refused, unreachable or errored read reaches nothing) and before the
+	// returns below — a completed page whose chapter number did not change
+	// still has to write. The transition is zero-versus-nonzero, not the
+	// stamp's value: a Series still completed keeps its original stamp, so the
+	// age #170 prints is "since the Site first said so"; one that stopped
+	// being completed is zeroed.
+	want := int64(0)
+	if facts.SiteCompleted {
+		want = p.Now().UnixMilli()
+	}
+	if (sr.SiteCompletedAt == 0) != (want == 0) {
+		if err := p.Store.SetSiteCompletedAt(sr.Site, sr.SeriesID, want); err != nil {
+			// Best-effort, like every poller write: never change the outcome
+			// word the pass counts.
+			log.Printf("latest poll %q: set site completed: %v", sr.Key(), err)
+		}
+	}
 	if !facts.HasLatest {
 		// Most likely a challenge page or a layout change. Either way the row is
 		// already stamped, so this waits out a rest instead of hot-looping.