fix: don't block the event loop with time.sleep in async load_checkpoint (#6135)

The export route's load_checkpoint waits for the training subprocess to
exit by calling time.sleep(0.5) in a loop (up to 30s) inside an async
function. time.sleep blocks the whole event loop, so every other request
to the server stalls for that duration. Use await asyncio.sleep(0.5),
matching the async pattern already used elsewhere in this file
(asyncio.to_thread, await asyncio.sleep).

Co-authored-by: Wasim Yousef Said <wasimysdev@gmail.com>

studio/backend/routes/export.py

@@ -78,8 +78,7 @@ async def load_checkpoint(
                 for _ in range(60):  # up to 30s
                     if not trn.is_training_active():
                         break
-                    import time
-                    time.sleep(0.5)
+                    await asyncio.sleep(0.5)
                 else:
                     logger.warning("Training subprocess did not exit within 30s, proceeding anyway")
         except Exception as e: