fixes generated image download

backend/app/config.py

@@ -13,6 +13,7 @@ class Settings(BaseSettings):
     GOOGLE_REDIRECT_URI: str = "http://localhost:4200"
     JWT_SECRET_KEY: str = "change-me-in-production"
     JWT_EXPIRE_MINUTES: int = 1440  # 24 hours
+    DOWNLOAD_TOKEN_MINUTES: int = 10  # short-lived signed download URLs
 
     # Visitor stats
     LOGS_DIR: str = "/logs"