Spec #136: Finished belongs to the Series — owner-owned poll gate, Lifecycle bucket dropped (#163)

Closes #136.

Spec #136 end to end: `finished` becomes a fact about the Series, written only by the owner, and the reader-facing Lifecycle bucket is gone.

## What landed

- **#157** — `series.finished_at bigint NOT NULL DEFAULT 0` plus the migration whose statement order is load-bearing (seed from the buckets, then flip them); both Lane queries lose the `HAVING COUNT(*) FILTER (WHERE b.status <> 'finished')` clause and gate on `finished_at = 0` instead, with the due-query/eligible-count force asymmetry kept deliberate and commented; `StatusFinished`, its API special-case 400, the web tab and the templates' Finished bucket deleted.
- **#158** — owner Finish control on the Series detail page: confirm-gated finish, instant un-finish, admin accent (never ember, nothing is destroyed), `Store.SetSeriesFinished`, the two routes behind the owner gate, and the state displayed on the list row without offering the control there.
- **#160** — reader side: derived `finished` bool on the flat Bookmark (`s.finished_at > 0`), rendered as a text-only label in both userscripts and on the web card; read-only inbound by omission from `Upsert`'s explicit `series` column list, same mechanism that already protects `cover`.
- **#161** — glossary and the stale Reader-count divergence note catch up.
- **#159** — `finished` joins the admin filter vocabulary (predicate `finished_at > 0`, label `Finished`, own aggregate count, figure last in the stats block as informational); the four clock-driven hygiene predicates (stale, never-checked, no-cover, no-chapter) exclude finished Series while unpollable, orphan and sighting-raised deliberately do not.

## Verification

`go vet ./...` and `go test ./...` green on the merged branch (Docker-backed, throwaway `postgres:17-alpine` per package). Each ticket also passed a two-axis review (spec + standards) on its own branch before merge.

Reviewed-on: #163
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/web/templates/series-detail.html

@@ -39,6 +39,22 @@
   <div class="field"><button class="ghost danger" hx-post="/admin/series/{{.Key}}/remove" hx-target="#detail-meta" hx-confirm="Removes this series and its stored cover. No Reader has it bookmarked; one re-bookmarking it recreates the row.">Remove</button></div>
 </div>
 {{end}}
+{{if .Finished}}
+<div class="dform">
+  <div class="field"><button type="button" class="ghost" hx-post="/admin/series/{{.Key}}/unfinish" hx-target="#detail-meta" hx-swap="outerHTML">Un-finish</button></div>
+</div>
+{{else}}
+<div class="dform">
+  <div class="field"><button type="button" class="ghost" onclick="document.getElementById('confirm-finish').hidden = false">Finish</button></div>
+  <div class="confirm-row calm" id="confirm-finish" role="group" aria-live="polite" hidden>
+    <span>Mark this Series finished?</span>
+    <div>
+      <button type="button" class="go" hx-post="/admin/series/{{.Key}}/finish" hx-target="#detail-meta" hx-swap="outerHTML">Finish</button>
+      <button type="button" onclick="document.getElementById('confirm-finish').hidden = true">Cancel</button>
+    </div>
+  </div>
+</div>
+{{end}}
 {{end}}
 
 {{/* series-detail-meta is the meta line, and the answer a Check now or
@@ -54,6 +70,7 @@
   <span>{{.Readers}} readers</span>
   {{if .Corrected}}<span class="mark">{{.Corrected}}</span>{{end}}
   {{if .Pending}}<span class="mark">{{.Requested}}</span>{{end}}
+  {{if .FinishedSince}}<span class="mark">{{.FinishedSince}}</span>{{end}}
   {{if .Unpollable}}<span class="mark">unpollable</span>{{end}}
   {{if .NoCover}}<span class="mark">no cover</span>{{end}}
   {{if .Orphan}}<span class="mark">orphan</span>{{end}}