refactor(web): share poll-state derivation; keep zebra parity across a row swap (#146)

Review round: pollState() unifies the CanPoll/Pending/Requested derivation
used by the list row and the detail page, and the row anchor carries its
band parity with the press (hx-vals) so the swapped answer keeps the
alternation.

backend/internal/web/admin_series_detail.go

@@ -96,6 +96,7 @@ func (h *Handler) adminSeriesByKey(site, seriesID string) (store.AdminSeries, bo
 // seriesDetailView shapes one AdminSeries row for display: every judgement in
 // Go, the template left to print strings and flags.
 func (h *Handler) seriesDetailView(a store.AdminSeries) seriesDetailView {
+	canPoll, pending, requested := pollState(a, time.Now())
 	v := seriesDetailView{
 		Key:            a.Key(),
 		Site:           a.Site,
@@ -107,11 +108,9 @@ func (h *Handler) seriesDetailView(a store.AdminSeries) seriesDetailView {
 		NoCover:        a.CoverAddress == "",
 		Orphan:         a.ReaderCount == 0,
 		SightingRaised: a.RaisedByReader,
-		CanPoll:        a.SeriesURL != "" && a.ReaderCount > 0,
-	}
-	if a.ForcePollAt > a.LatestCheckedAt {
-		v.Pending = true
-		v.Requested = requestedAge(time.Now(), a.ForcePollAt)
+		CanPoll:        canPoll,
+		Pending:        pending,
+		Requested:      requested,
 	}
 	if a.LatestChapterNum == nil {
 		v.Chapter = "—"