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

.sqlx/query-f51e2addb844a3d386ba582c71f41c2bfbbec508109bed2e7a460ffcc2b5cdf1.json

@@ -0,0 +1,12 @@
+{
+  "db_name": "SQLite",
+  "query": "UPDATE seasons\n         SET last_pack_search_at = strftime('%Y-%m-%dT%H:%M:%fZ', 'now'),\n             updated_at = strftime('%Y-%m-%dT%H:%M:%fZ', 'now')\n         WHERE id = ?",
+  "describe": {
+    "columns": [],
+    "parameters": {
+      "Right": 1
+    },
+    "nullable": []
+  },
+  "hash": "f51e2addb844a3d386ba582c71f41c2bfbbec508109bed2e7a460ffcc2b5cdf1"
+}