Speed up ragflow server (#14894)

### What problem does this PR solve?

Speed up ragflow server

### Type of change

- [ ] Refactoring

api/ragflow_server.py

@@ -19,8 +19,13 @@ print("Start RAGFlow server...")
 import time
 start_ts = time.time()
 
-import logging
 import os
+
+# LiteLLM fetches a model cost map from GitHub during import unless this is set.
+# The API server should not block startup on external network access.
+os.environ.setdefault("LITELLM_LOCAL_MODEL_COST_MAP", "True")
+
+import logging
 import signal
 import sys
 import threading