feat: progress reporting with chunk context + live job persistence

- ProgressEvent::Progress now carries chunk index and total count
- SsePayload::Progress gains chunk / chunks_total fields
  → SSE clients can show 'chunk N/51' instead of bare percent
- process_job persists job.progress to storage at each chunk boundary
  → GET /jobs/:id now shows live progress (not stuck at 0)
- Emits Progress event at chunk START (boundary event), not just on
  whisper's internal callback
- entropy_thold raised to 3.5 (catches medium-phrase loops; triggers
  whisper's own temperature-retry instead of silent repetition)
- no_speech_thold removed (confirmed // TODO: not implemented in
  whisper.cpp source; was a no-op)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/routes/jobs.rs

@@ -192,9 +192,9 @@ pub async fn stream_job(
 
     let sse_stream: SseStream = Box::pin(BroadcastStream::new(rx).filter_map(|msg| async move {
         let event = match msg {
-            Ok(ProgressEvent::Progress(p)) => {
+            Ok(ProgressEvent::Progress { percent, chunk, total }) => {
                 let payload = serde_json::to_string(
-                    &crate::models::SsePayload::Progress { percent: p }
+                    &crate::models::SsePayload::Progress { percent, chunk, chunks_total: total }
                 ).ok()?;
                 Event::default().event("progress").data(payload)
             }