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/lib.rs

@@ -7,11 +7,43 @@ pub mod transcribe;
 
 use std::sync::Arc;
 
+use axum::extract::FromRef;
 use axum::Router;
 
 use config::Config;
+use transcribe::TranscribeSender;
 
-/// Build the application router. Used by main.rs and integration tests.
-pub fn create_router(config: Arc<Config>) -> Router {
-    routes::api_router().with_state(config)
+/// Shared application state. Clonable — all fields are cheap to clone
+/// (`Arc` and `mpsc::Sender`).
+#[derive(Clone)]
+pub struct AppState {
+    pub config: Arc<Config>,
+    pub transcribe_tx: TranscribeSender,
+}
+
+impl FromRef<AppState> for Arc<Config> {
+    fn from_ref(state: &AppState) -> Self {
+        state.config.clone()
+    }
+}
+
+impl FromRef<AppState> for TranscribeSender {
+    fn from_ref(state: &AppState) -> Self {
+        state.transcribe_tx.clone()
+    }
+}
+
+/// Test/simple entrypoint: jobs pushed into the transcribe channel are dropped
+/// because the receiver is not retained. Use [`create_router_with_state`] from
+/// `main.rs` where a real worker owns the receiver.
+pub fn create_router(config: Arc<Config>) -> Router {
+    let (tx, _rx) = transcribe::channel();
+    create_router_with_state(AppState {
+        config,
+        transcribe_tx: tx,
+    })
+}
+
+pub fn create_router_with_state(state: AppState) -> Router {
+    routes::api_router().with_state(state)
 }