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/app.html

@@ -71,10 +71,6 @@
        {{if eq .Tab "archived"}}aria-current="page"{{end}}
        hx-get="{{.ListURL "archived"}}" hx-target="#list" hx-swap="innerHTML"
        hx-push-url="{{.PageURL "archived"}}" hx-on::after-request="setActiveTab(this)">Archived</a>
-    <a href="{{.PageURL "finished"}}" class="{{if eq .Tab "finished"}}active{{end}}"
-       {{if eq .Tab "finished"}}aria-current="page"{{end}}
-       hx-get="{{.ListURL "finished"}}" hx-target="#list" hx-swap="innerHTML"
-       hx-push-url="{{.PageURL "finished"}}" hx-on::after-request="setActiveTab(this)">Finished</a>
   </nav>
   </div>