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/admin_series_detail.go

@@ -56,6 +56,14 @@ type seriesDetailView struct {
 	// Series no Reader holds can be removed, so the owner is never offered a
 	// button the database will always refuse.
 	CanRemove bool
+	// Finished is the owner's finish stamp rendered for the control: while it
+	// stands, the page offers the instant Un-finish, not the confirm-gated
+	// Finish (issue #158).
+	Finished bool
+	// FinishedSince is the "finished <age> ago" line, "" while no finish
+	// stands. It rides the meta fragment both presses swap, so the answer
+	// itself shows how long the Series has been finished.
+	FinishedSince string
 }
 
 // adminSeriesDetail renders one Series' page, keyed by the composite
@@ -141,6 +149,8 @@ func (h *Handler) seriesDetailView(a store.AdminSeries) seriesDetailView {
 		v.Checked = since(time.Now(), time.UnixMilli(a.LatestCheckedAt))
 	}
 	v.Corrected = correctedAge(time.Now(), a.LatestCorrectedAt)
+	v.Finished = a.FinishedAt != 0
+	v.FinishedSince = finishedAge(time.Now(), a.FinishedAt)
 
 	// Provenance: the actor class behind the current number, evaluated in the
 	// order the classes outrank one another — the owner's stamp, which a
@@ -171,3 +181,13 @@ func correctedAge(now time.Time, at int64) string {
 	}
 	return "corrected " + since(now, time.UnixMilli(at))
 }
+
+// finishedAge is the finish marker's text: "finished <age> ago" while the
+// stamp is set, "" when zero — zero means never finished, and the reversal
+// (un-finish) must not read as history after a press (issue #158).
+func finishedAge(now time.Time, at int64) string {
+	if at == 0 {
+		return ""
+	}
+	return "finished " + since(now, time.UnixMilli(at))
+}