fix: create .env file if missing when saving API keys

Docker users don't have a .env file by default, so the settings
page silently failed to save keys. Now creates it automatically.

backend/services/api_settings.py

@@ -167,7 +167,7 @@ def update_api_key(env_key: str, new_value: str) -> bool:
         return False
 
     if not ENV_PATH.exists():
-        return False
+        ENV_PATH.write_text("", encoding="utf-8")
 
     # Update os.environ immediately
     os.environ[env_key] = new_value