enh: enable message rating setting

backend/config.py

@@ -842,6 +842,12 @@ ENABLE_COMMUNITY_SHARING = PersistentConfig(
     os.environ.get("ENABLE_COMMUNITY_SHARING", "True").lower() == "true",
 )
 
+ENABLE_MESSAGE_RATING = PersistentConfig(
+    "ENABLE_MESSAGE_RATING",
+    "ui.enable_message_rating",
+    os.environ.get("ENABLE_MESSAGE_RATING", "True").lower() == "true",
+)
+
 
 def validate_cors_origins(origins):
     for origin in origins: