feat: add LATEST_VERSION config for client update detection

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cloud_backend/config.py

@@ -10,6 +10,7 @@ class Settings(BaseSettings):
     ACCESS_TOKEN_EXPIRE_MINUTES: int = 60
     ADMIN_USERNAME: str = "sysadmin"
     ADMIN_PASSWORD: str = "changeme"
+    LATEST_VERSION: str = "0.0.0"
 
     model_config = {"env_file": str(_HERE / ".env")}