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-0ca521a8dcb979cc90f5823eb3311d6a3ec1613976c52a8a7f8225e1dc06d162.json

@@ -0,0 +1,28 @@
+{
+  "db_name": "SQLite",
+  "query": "SELECT count(*)        AS \"failures!: i64\",\n                  max(grabbed_at) AS \"last_failed_at?: String\"\n           FROM   grabs\n           WHERE  target_kind = 'season' AND target_id = ? AND state = 'failed'",
+  "describe": {
+    "columns": [
+      {
+        "name": "failures!: i64",
+        "ordinal": 0,
+        "type_info": "Integer",
+        "origin": "Expression"
+      },
+      {
+        "name": "last_failed_at?: String",
+        "ordinal": 1,
+        "type_info": "Text",
+        "origin": "Expression"
+      }
+    ],
+    "parameters": {
+      "Right": 1
+    },
+    "nullable": [
+      false,
+      true
+    ]
+  },
+  "hash": "0ca521a8dcb979cc90f5823eb3311d6a3ec1613976c52a8a7f8225e1dc06d162"
+}