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

src/api/mod.rs

@@ -1,5 +1,6 @@
 mod indexer_controller;
 mod metadata_controller;
+mod sync_controller;
 mod torrent_controller;
 
 use axum::{
@@ -17,6 +18,7 @@ use crate::AppState;
 pub fn routes(state: AppState) -> Router {
     Router::new()
         .route("/health", get(health))
+        .route("/reload", post(reload))
         .route("/tracks", get(list_tracks))
         .route("/tracks", post(create_track))
         .route("/tracks/{id}", get(get_track))
@@ -26,6 +28,7 @@ pub fn routes(state: AppState) -> Router {
         .nest("/indexers", indexer_controller::routes())
         .nest("/torrents", torrent_controller::routes())
         .nest("/metadata", metadata_controller::routes())
+        .nest("/sync", sync_controller::routes())
         .with_state(state)
 }
 
@@ -60,6 +63,27 @@ async fn health(State(state): State<AppState>) -> Json<Health> {
     })
 }
 
+#[derive(serde::Serialize)]
+struct ReloadResponse {
+    success: bool,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    error: Option<String>,
+}
+
+async fn reload(State(state): State<AppState>) -> Json<ReloadResponse> {
+    let mut state = state.write().await;
+    match state.reload().await {
+        Ok(()) => Json(ReloadResponse {
+            success: true,
+            error: None,
+        }),
+        Err(e) => Json(ReloadResponse {
+            success: false,
+            error: Some(e),
+        }),
+    }
+}
+
 async fn list_tracks(State(state): State<AppState>) -> Json<Vec<Track>> {
     let state = state.read().await;
     Json(state.aggregator.get_all().to_vec())