fix: remove stale _decimal_str ref, add context manager to HTTP clients

- UpstreamClient & Sub2ApiWebsiteClient: add __enter__/__exit__
- Convert all call sites to `with Client(...) as c:` pattern
- Remove unused `upstream_name`/`upstream_base_url` locals in scheduler
- Fix stale _decimal_str→decimal_string in _rate_from_group

backend/app/services/website_client.py

@@ -105,6 +105,12 @@ class Sub2ApiWebsiteClient:
     def close(self) -> None:
         self._client.close()
 
+    def __enter__(self) -> Sub2ApiWebsiteClient:
+        return self
+
+    def __exit__(self, *args: Any) -> None:
+        self.close()
+
     def _url(self, path: str) -> str:
         prefix = f"/{self.api_prefix}" if self.api_prefix else ""
         return f"{self.base_url}{prefix}/{path.lstrip('/')}"