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.
