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/test_upstream.py

@@ -6,31 +6,31 @@ from app.services.upstream_client import UpstreamClient
 logging.basicConfig(level=logging.DEBUG)
 
 def main():
-    client = UpstreamClient(
+    with UpstreamClient(
         base_url="http://170.106.100.210:55555",
         api_prefix="",
         auth_type="bearer",
         auth_config={"token": ""},  # We don't have token, but /api/group/ in some new-api may be open, or fail with 401
         timeout=10.0,
-    )
-    try:
-        groups = client.get_available_groups("/api/group/")
-        print("Groups:", groups)
-    except Exception as e:
-        print("Groups Error:", e)
+    ) as client:
+        try:
+            groups = client.get_available_groups("/api/group/")
+            print("Groups:", groups)
+        except Exception as e:
+            print("Groups Error:", e)
 
-    try:
-        rates = client.get_group_rates("/api/option/?key=GroupRatio")
-        print("Rates:", rates)
-    except Exception as e:
-        print("Rates Error:", e)
+        try:
+            rates = client.get_group_rates("/api/option/?key=GroupRatio")
+            print("Rates:", rates)
+        except Exception as e:
+            print("Rates Error:", e)
 
-    try:
-        from app.services.upstream_client import _extract_rates_map, _unwrap_list
-        print("Unwrapped Groups:", _unwrap_list(groups))
-        print("Extracted Rates:", _extract_rates_map(rates))
-    except Exception as e:
-        pass
+        try:
+            from app.services.upstream_client import _extract_rates_map, _unwrap_list
+            print("Unwrapped Groups:", _unwrap_list(groups))
+            print("Extracted Rates:", _extract_rates_map(rates))
+        except Exception as e:
+            pass
 
 if __name__ == "__main__":
     main()