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/routes/mod.rs

@@ -3,14 +3,12 @@ mod health;
 mod upload;
 mod web;
 
-use std::sync::Arc;
-
 use axum::routing::{get, post};
 use axum::Router;
 
-use crate::config::Config;
+use crate::AppState;
 
-pub fn api_router() -> Router<Arc<Config>> {
+pub fn api_router() -> Router<AppState> {
     Router::new()
         .route("/api/health", get(health::handle_health))
         .route("/api/debug/whoami", get(debug::handle_whoami))

server/src/routes/upload.rs

@@ -1,15 +1,18 @@
 use std::path::{Path, PathBuf};
 
-use axum::extract::Multipart;
+use axum::extract::{Multipart, State};
 use axum::Json;
-use tracing::info;
+use tokio::sync::mpsc::error::TrySendError;
+use tracing::{info, warn};
 
 use crate::auth::AuthenticatedUser;
 use crate::error::AppError;
 use crate::models::{AckResponse, AckStatus};
+use crate::transcribe::{TranscribeJob, TranscribeSender};
 
 pub async fn handle_upload(
     user: AuthenticatedUser,
+    State(transcribe_tx): State<TranscribeSender>,
     mut multipart: Multipart,
 ) -> Result<Json<AckResponse>, AppError> {
     let mut case_id: Option<String> = None;
@@ -81,6 +84,18 @@ pub async fn handle_upload(
         "Recording received"
     );
 
+    // Enqueue transcription. Failure here is non-fatal: on restart the recovery
+    // scan will re-enqueue any .m4a without a .transcript.txt sibling.
+    match transcribe_tx.try_send(TranscribeJob { audio_path: file_path.clone() }) {
+        Ok(()) => {}
+        Err(TrySendError::Full(_)) => {
+            warn!(file = %file_path.display(), "Transcription queue full; will be picked up on restart");
+        }
+        Err(TrySendError::Closed(_)) => {
+            warn!(file = %file_path.display(), "Transcription queue closed (no worker)");
+        }
+    }
+
     Ok(Json(AckResponse {
         case_id,
         recorded_at,