Fix: Prevent Flask hot reload from hanging due to early thread startup (#7966)

**Fix: Prevent Flask hot reload from hanging due to early thread
startup**

### What problem does this PR solve?

When running the Flask server with `use_reloader=True` (enabled during
debug mode), modifying a Python source file would trigger a reload
detection (`Detected change in ...`), but the application would hang
instead of restarting cleanly.

This was caused by the `update_progress` background thread being started
**too early**, often within the main module scope.
This issue was reported in
[#7498](https://github.com/infiniflow/ragflow/issues/7498).

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
---

**Summary of changes:**
- Wrapped `update_progress` launch in a `threading.Timer` with delay to
avoid premature thread execution.
- Marked thread as `daemon=True` to avoid blocking process exit.
- Added `WERKZEUG_RUN_MAIN` environment check to ensure background
threads only run in the reloader child process (the actual Flask app).
- Retained original behavior in production mode (`debug=False`).

---------

Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

api/ragflow_server.py

@@ -28,7 +28,6 @@ import signal
 import sys
 import time
 import traceback
-from concurrent.futures import ThreadPoolExecutor
 import threading
 import uuid
 
@@ -125,8 +124,16 @@ if __name__ == '__main__':
     signal.signal(signal.SIGINT, signal_handler)
     signal.signal(signal.SIGTERM, signal_handler)
 
-    thread = ThreadPoolExecutor(max_workers=1)
-    thread.submit(update_progress)
+    def delayed_start_update_progress():
+        logging.info("Starting update_progress thread (delayed)")
+        t = threading.Thread(target=update_progress, daemon=True)
+        t.start()
+
+    if RuntimeConfig.DEBUG:
+        if os.environ.get("WERKZEUG_RUN_MAIN") == "true":
+            threading.Timer(1.0, delayed_start_update_progress).start()
+    else:
+        threading.Timer(1.0, delayed_start_update_progress).start()
 
     # start http server
     try: