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-20c1cbcd9cc317b148357ff64e967e30032a438c4815816323cf62896398d2a2.json

@@ -0,0 +1,33 @@
+{
+  "db_name": "SQLite",
+  "query": "SELECT   e.air_date,\n                    EXISTS (\n                        SELECT 1 FROM media_files f\n                        WHERE  f.owner_kind = 'episode' AND f.owner_id = e.id\n                    ) AS \"has_file!: bool\"\n           FROM     episodes e\n           WHERE    e.season_id = ?",
+  "describe": {
+    "columns": [
+      {
+        "name": "air_date",
+        "ordinal": 0,
+        "type_info": "Text",
+        "origin": {
+          "Table": {
+            "table": "episodes",
+            "name": "air_date"
+          }
+        }
+      },
+      {
+        "name": "has_file!: bool",
+        "ordinal": 1,
+        "type_info": "Integer",
+        "origin": "Expression"
+      }
+    ],
+    "parameters": {
+      "Right": 1
+    },
+    "nullable": [
+      true,
+      false
+    ]
+  },
+  "hash": "20c1cbcd9cc317b148357ff64e967e30032a438c4815816323cf62896398d2a2"
+}