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>

test/testcases/test_web_api/test_system_app/test_apps_init_unit.py

@@ -49,6 +49,7 @@ def _load_apps_module(monkeypatch):
 
     settings_mod = ModuleType("common.settings")
     settings_mod.SECRET_KEY = "test-secret-key"
+    settings_mod.get_secret_key = lambda: "test-secret-key"
     settings_mod.init_settings = lambda: None
     settings_mod.decrypt_database_config = lambda name=None: {}
     monkeypatch.setitem(sys.modules, "common.settings", settings_mod)