Fix secret key inconsistency cross the RAGFlow servers (#14591)

### What problem does this PR solve?

A and B, two API servers and a REDIS server.
If A and REDIS restart, B will hold the obsolete secret key and will
lead to error.

TODO:
app.config['SECRET_KEY'] and app.secret_key still hold obsolete secret
key.

### Type of change

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

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

admin/server/auth.py

@@ -58,7 +58,7 @@ def setup_auth(login_manager):
                     return None
 
                 # Decode JWT to get the UUID access_token
-                jwt = Serializer(secret_key=settings.SECRET_KEY)
+                jwt = Serializer(secret_key=settings.get_secret_key())
                 access_token = str(jwt.loads(jwt_token))
 
                 if not access_token or not access_token.strip():