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-2759a3a5fb0ca9d37560b8d89922c7c5618b25843192ffa28b9c13eacd0b9c26.json

@@ -0,0 +1,26 @@
+{
+  "db_name": "SQLite",
+  "query": "SELECT last_pack_search_at FROM seasons WHERE id = ?",
+  "describe": {
+    "columns": [
+      {
+        "name": "last_pack_search_at",
+        "ordinal": 0,
+        "type_info": "Text",
+        "origin": {
+          "Table": {
+            "table": "seasons",
+            "name": "last_pack_search_at"
+          }
+        }
+      }
+    ],
+    "parameters": {
+      "Right": 1
+    },
+    "nullable": [
+      true
+    ]
+  },
+  "hash": "2759a3a5fb0ca9d37560b8d89922c7c5618b25843192ffa28b9c13eacd0b9c26"
+}