refine db connection (#1494)

### What problem does this PR solve?



### Type of change


- [x] Refactoring

api/db/db_models.py

 def close_connection():
     try:
         if DB:
-            DB.close()
+            DB.close_stale(age=30)
     except Exception as e:
         LOGGER.exception(e)
 

api/ragflow_server.py

         werkzeug_logger = logging.getLogger("werkzeug")
         for h in access_logger.handlers:
             werkzeug_logger.addHandler(h)
-        run_simple(hostname=HOST, port=HTTP_PORT, application=app, processes=5, use_reloader=RuntimeConfig.DEBUG, use_debugger=RuntimeConfig.DEBUG)
+        run_simple(hostname=HOST, port=HTTP_PORT, application=app, threaded=True, use_reloader=RuntimeConfig.DEBUG, use_debugger=RuntimeConfig.DEBUG)
     except Exception:
         traceback.print_exc()
         os.kill(os.getpid(), signal.SIGKILL)

rag/llm/__init__.py

     "BAAI": DefaultEmbedding,
     "Mistral": MistralEmbed,
     "Bedrock": BedrockEmbed,
-    "Gemini":GeminiEmbed,
+    "Gemini":GeminiEmbed
 }