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>

CONTEXT.md

@@ -95,7 +95,7 @@ _Avoid_: run, cycle, tick, batch, poll history
 **Forced Poll**:
 A Poll the owner asks for by hand instead of waiting for the Series's turn. It jumps its
 Lane's queue and ignores every waiting rule — the rest between Polls, a Sighting standing
-in for a check, a Series only finished Readers hold — but never overrules a Site that is
+in for a check, a finished Series — but never overrules a Site that is
 refusing us, the Lane's spacing between fetches, or a Series with no page to fetch. Asked
 for by marking the Series, never by commanding the poller, so it happens on the Lane's
 next pass rather than at the moment of asking.
@@ -149,10 +149,22 @@ accent is permitted to signal.
 _Avoid_: unread, update available
 
 **Lifecycle bucket**:
-Which of three mutually exclusive states a Bookmark sits in — reading, archived, or
-finished. A Bookmark is in exactly one. Orthogonal to being a favourite.
+Which of the two states a Bookmark sits in — reading or archived. A Bookmark is in exactly
+one. Orthogonal to being a favourite. Finished is not a bucket: it is a fact about the
+Series (see `series.finished_at`), owned by the owner and stamped once, and every Bookmark
+on a finished Series is archived.
 _Avoid_: state, status (as a domain word), list
 
+**Finished Series**:
+A Series the owner has marked finished, stamped once in `series.finished_at`
+(epoch ms, zero means not finished). The owner is its only writer — no
+adapter, no Reader, no Poll can set it — and a Forced Poll reads a finished
+Series once for that pass and never clears the flag. It is a fact about the
+Series, not a Bookmark bucket: every Bookmark on a finished Series is
+archived, the Lane stops polling it (the due gate reads `finished_at = 0`),
+and Readers see a label and nothing more.
+_Avoid_: completed, done, dropped, shelved (that is Archived), ended
+
 **Favourite**:
 A reader's manual pin on a Bookmark. Orthogonal to the Lifecycle bucket, and never a
 reason to reorder the list.