Merge #169: The pass remembers when a Site called a work completed

# Conflicts:
#	backend/internal/latest/poller_test.go

backend/internal/latest/poller.go

@@ -719,6 +719,25 @@ 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.
+	stamp := int64(0)
+	if facts.SiteCompleted {
+		stamp = p.Now().UnixMilli()
+	}
+	if (sr.SiteCompletedAt == 0) != (stamp == 0) {
+		if err := p.Store.SetSiteCompletedAt(sr.Site, sr.SeriesID, stamp); 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.