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

Cargo.toml

@@ -23,6 +23,7 @@ thiserror = "2"
 url = "2"
 roxmltree = "0.20"
 base64 = "0.22"
+chrono = { version = "0.4", features = ["serde"] }
 clap = { version = "4", features = ["derive"] }
 tonic = "0.12"
 prost = "0.13"