feat: remove files for one season or episode

DELETE /api/series/{id}/seasons/{n}/files and
DELETE /api/episodes/{id}/files unlink what the scope covers, drop the
matching media_files rows and clear wanted, in one action. 204 on
success, 404 for an unknown season or episode, and a scope with nothing
on disk still clears intent.

Season and episode rows stay: TMDB owns that metadata and the next
refresh would recreate them.

The three scopes share one unlink path. A whole series still resolves to
its title folder (§7.4, atomic); a season or episode resolves to the
recorded file and nothing else, so a narrow call cannot reach a sibling.
The intent clear goes through arr_core::tracking::apply_tracked(false),
the same §4.1 rule #171 landed.

An episode whose file just went is set back to 'missing' when it was
'available', matching what a failed import already does. Closes #174.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.sqlx/query-e5c40363c81cebf7026b49699981218e15b57d565039f3de2d48b320d3cec92b.json

@@ -0,0 +1,20 @@
+{
+  "db_name": "SQLite",
+  "query": "SELECT mf.path AS \"path!: String\"\n                   FROM media_files mf\n                   JOIN episodes e ON mf.owner_kind = 'episode' AND e.id = mf.owner_id\n                   WHERE e.season_id = ?",
+  "describe": {
+    "columns": [
+      {
+        "name": "path!: String",
+        "ordinal": 0,
+        "type_info": "Text"
+      }
+    ],
+    "parameters": {
+      "Right": 1
+    },
+    "nullable": [
+      false
+    ]
+  },
+  "hash": "e5c40363c81cebf7026b49699981218e15b57d565039f3de2d48b320d3cec92b"
+}