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/transcribe/mod.rs

@@ -1,2 +1,21 @@
+use std::path::PathBuf;
+
+use tokio::sync::mpsc;
+
 pub mod ffmpeg;
 pub mod whisper;
+pub mod worker;
+
+pub const QUEUE_DEPTH: usize = 64;
+
+#[derive(Debug, Clone)]
+pub struct TranscribeJob {
+    pub audio_path: PathBuf,
+}
+
+pub type TranscribeSender = mpsc::Sender<TranscribeJob>;
+pub type TranscribeReceiver = mpsc::Receiver<TranscribeJob>;
+
+pub fn channel() -> (TranscribeSender, TranscribeReceiver) {
+    mpsc::channel(QUEUE_DEPTH)
+}