feat: Add transcription worker and state management

Introduce an `AppState` struct to hold shared application state,
including configuration and the transcription sender.
Modify `create_router` and `create_router_with_state` to use `AppState`.
Update `main.rs` to initialize the transcription worker and pass the
`AppState` to the router.
Implement the transcription worker logic in
`server/src/transcribe/worker.rs`, handling job queues, FFmpeg remuxing,
and Whisper transcription.
Update `server/src/auth.rs` to use `Arc<Config>` from the shared state.
Modify `server/src/routes/upload.rs` to enqueue transcription jobs and
handle potential errors.
Add `TranscribeJob` and channel types in `server/src/transcribe/mod.rs`.

server/src/main.rs

@@ -9,6 +9,8 @@ use tracing_subscriber::util::SubscriberInitExt;
 use tracing_subscriber::EnvFilter;
 
 use doctate_server::config::Config;
+use doctate_server::transcribe;
+use doctate_server::AppState;
 
 #[tokio::main]
 async fn main() {
@@ -37,10 +39,26 @@ async fn main() {
         )
         .init();
 
+    // Transcription pipeline: shared HTTP client + worker task.
+    let (transcribe_tx, transcribe_rx) = transcribe::channel();
+    let http_client = reqwest::Client::builder()
+        .build()
+        .expect("reqwest client build failed");
+    tokio::spawn(transcribe::worker::run(
+        transcribe_rx,
+        config.clone(),
+        http_client,
+    ));
+
+    let state = AppState {
+        config: config.clone(),
+        transcribe_tx,
+    };
+
     let addr = format!("0.0.0.0:{}", config.server_port);
     info!(port = config.server_port, "Server starting");
 
-    let app = doctate_server::create_router(config)
+    let app = doctate_server::create_router_with_state(state)
         .layer(TraceLayer::new_for_http())
         .layer(RequestBodyLimitLayer::new(50 * 1024 * 1024)); // 50 MB