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/web/templates/list.html

@@ -14,8 +14,6 @@
   <div class="empty"><strong>Nothing new.</strong><p>Every series is caught up to its latest chapter.</p></div>
 {{else if eq .Tab "archived"}}
   <div class="empty"><strong>Nothing archived.</strong><p>Shelve a series to park it here — it keeps getting checked for new chapters.</p></div>
-{{else if eq .Tab "finished"}}
-  <div class="empty"><strong>Nothing finished yet.</strong><p>Mark a series finished and it moves out of your reading list.</p></div>
 {{else if .EmptyLibrary}}
   {{/* Nothing in either library, so the links are the only thing this page can
        usefully say. Both scripts: the two libraries are separate installs. */}}