Fix Start button not updating: unblock the event loop

start_transcription() blocks up to 15s waiting for the Deepgram
WebSocket to connect. Running it synchronously in the async endpoint
blocked the entire uvicorn event loop, preventing:
- pollStatus from completing (frozen HTTP request)
- WebSocket broadcasts from being sent
- Any other API requests from being handled

Fix: run start/stop/reload in thread pool via run_in_executor so
the event loop stays responsive during long-running operations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/api_server.py

@@ -151,14 +151,24 @@ class APIServer:
 
         @app.post("/api/start")
         async def start_transcription():
-            success, message = ctrl.start_transcription()
+            import asyncio
+            # Run in thread pool to avoid blocking the event loop
+            # (start_recording can block up to 15s waiting for Deepgram WS)
+            loop = asyncio.get_event_loop()
+            success, message = await loop.run_in_executor(
+                None, ctrl.start_transcription
+            )
             if not success:
                 raise HTTPException(status_code=400, detail=message)
             return {"status": "ok", "message": message}
 
         @app.post("/api/stop")
         async def stop_transcription():
-            success, message = ctrl.stop_transcription()
+            import asyncio
+            loop = asyncio.get_event_loop()
+            success, message = await loop.run_in_executor(
+                None, ctrl.stop_transcription
+            )
             if not success:
                 raise HTTPException(status_code=400, detail=message)
             return {"status": "ok", "message": message}
@@ -223,7 +233,11 @@ class APIServer:
 
         @app.post("/api/reload-engine")
         async def reload_engine():
-            success, message = ctrl.reload_engine()
+            import asyncio
+            loop = asyncio.get_event_loop()
+            success, message = await loop.run_in_executor(
+                None, ctrl.reload_engine
+            )
             if not success:
                 raise HTTPException(status_code=500, detail=message)
             return {"status": "ok", "message": message}