Spec #136: finished becomes a Series fact, Lane gate rewritten on it (#157)

finished_at lands on series (0016 seeds it from the pre-flip finished
bookmarks, then flips those bookmarks to archived), the poll gate and
eligible count read the flag instead of a per-Reader vote, PUT rejects
the finished status like any unknown value, and the web UI drops the
Finished tab, badge and finish button. Novel merge rank is archived >
reading. Per-Reader disagreement (one Reader keeps a finished Series in
reading forever) is what the flag repairs; the cutover keeps polling
state unchanged for every Series.

backend/internal/store/admin.go

@@ -157,9 +157,9 @@ func adminFilter(f SeriesFilter) (where, having string, args []any, err error) {
 // SeriesPage returns one page of the Series matching the filter, least
 // recently checked first. The LEFT JOIN to Bookmarks is what surfaces the
 // orphans that hygiene has to find — an inner join would hide them, exactly
-// as the Lane's join does. ReaderCount is a plain count of every Bookmark on
-// the Series, which knowingly disagrees with the two Lane queries for as long
-// as the finished lifecycle bucket exists (#140).
+// as the Lane's join does. Every bookmark keeps its Series polled now that
+// finished is a Series flag, so this plain ReaderCount agrees with the Lane
+// queries (issue #157).
 //
 // The tie-break is mandatory, not decorative: every unpollable Series shares a
 // zero check stamp, so ordering on that column alone gives no stable page