Address review nits on #169: stamp name, migration comment (#169)

backend/internal/latest/poller.go

@@ -686,12 +686,12 @@ func (p *Poller) checkOne(ctx context.Context, sr store.Series) (outcome readOut
 	// 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)
+	stamp := int64(0)
 	if facts.SiteCompleted {
-		want = p.Now().UnixMilli()
+		stamp = p.Now().UnixMilli()
 	}
-	if (sr.SiteCompletedAt == 0) != (want == 0) {
-		if err := p.Store.SetSiteCompletedAt(sr.Site, sr.SeriesID, want); err != nil {
+	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)