feat: add artist sync flow and stub torrent client

- Add DownloadService to orchestrate metadata → indexer → torrent flow
- Add POST /api/sync/artist endpoint for syncing artist albums
- Add StubTorrentClient for testing (logs requests to file)
- Refactor TorrentConfig to tagged enum (client_type: qbittorrent|stub|none)
- Add POST /api/reload endpoint for hot config reload
- Add chrono dependency for timestamps

config.example.yaml

@@ -13,8 +13,19 @@ indexers:
     url: "http://localhost:9117"
     api_key: "your-jackett-api-key"
 
+# Torrent client - choose one of: qbittorrent, stub, none
 torrent:
-  qbittorrent:
-    url: "http://localhost:8080"
-    username: "admin"
-    password: "changeme"
+  client_type: qbittorrent
+  url: "http://localhost:8080"
+  username: "admin"
+  password: "changeme"
+
+# Alternative: stub client for testing
+# torrent:
+#   client_type: stub
+#   log_path: "/tmp/torrent-stub.log"
+#   save_path: "/tmp/downloads"
+
+# Alternative: no torrent client
+# torrent:
+#   client_type: none