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/store/migrations/0016_finished_series.sql

@@ -0,0 +1,24 @@
+-- finished_at is "the owner marked this Series finished" (#157): epoch ms,
+-- zero means not finished, and it doubles as the undo (write zero). The poll
+-- gate reads it — a Series is polled only while finished_at = 0 — never a
+-- bookmark's status.
+ALTER TABLE series ADD COLUMN finished_at bigint NOT NULL DEFAULT 0;
+
+-- Column first, seed second, flip third — the order is load-bearing: a seed
+-- that ran after the flip would read the buckets it just destroyed, declare
+-- nothing finished, and silently resume polling on Series nobody chose to
+-- resume. The seed mirrors the pre-cutover due gate exactly: a Series stays
+-- polled while any bookmark is outside the finished bucket, so a Series whose
+-- every bookmark sits in it is stamped, one click from being read again
+-- afterwards. The stamp is the only memory of the bucket the flip is about to
+-- erase.
+UPDATE series s SET finished_at = (EXTRACT(EPOCH FROM now()) * 1000)::bigint
+ WHERE EXISTS (SELECT 1 FROM bookmarks b
+                WHERE b.site = s.site AND b.series_id = s.series_id)
+   AND NOT EXISTS (SELECT 1 FROM bookmarks b
+                    WHERE b.site = s.site AND b.series_id = s.series_id
+                      AND b.status <> 'finished');
+
+-- The Lifecycle bucket is gone; a finished bookmark is an archived one. The
+-- flip must come after the seed, which still reads the bucket.
+UPDATE bookmarks SET status = 'archived' WHERE status = 'finished';