feat(web): name the season deck's real state

An empty season deck was three truths wearing one message, and the one
it chose to blame was wrong: a season on the per-episode lane sat on
"sweeping indexers…" for the full wait and then blamed a backoff for a
pack search that was never going to run.

`GET /api/series/{id}/seasons/{n}/pack-state` says which lane the
season takes and why, from `season_grab_reason` in arr-core, plus the
failed-pack tally and when #181's window reopens. Seasons gain
`last_pack_search_at`, written only by a season-scoped sweep, so a
pack search that ran and found nothing is a settled answer rather than
a pending one.

The deck then says the true thing in each case, and a season held off
the pack lane by a failure offers the retry that waives its window.

Refs #182

crates/arr-db/migrations/0023_season_pack_search.sql

@@ -0,0 +1,6 @@
+-- #182. The season release deck cannot tell a queued pack sweep from one
+-- that ran and found nothing, so it blames backoff for both. Only the season
+-- knows when its own pack search last completed: episodes' `last_searched_at`
+-- moves for reasons that have nothing to do with the pack lane, and a season
+-- on the per-episode lane never touches the pack search at all.
+ALTER TABLE seasons ADD COLUMN last_pack_search_at TEXT;