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/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';