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/routers/websites.py

@@ -188,7 +188,8 @@ def test_website(wid: int, db: Session = Depends(get_db), _=Depends(get_current_
     if not row:
         raise HTTPException(404, "website not found")
     try:
-        groups = _client(row).get_groups(row.groups_endpoint)
+        with _client(row) as c:
+            groups = c.get_groups(row.groups_endpoint)
         row.last_status = "healthy"
         row.last_error = None
         row.last_checked_at = datetime.now(timezone.utc)
@@ -208,7 +209,8 @@ def list_website_groups(wid: int, db: Session = Depends(get_db), _=Depends(get_c
     if not row:
         raise HTTPException(404, "website not found")
     try:
-        return _client(row).get_groups(row.groups_endpoint)
+        with _client(row) as c:
+            return c.get_groups(row.groups_endpoint)
     except Exception as exc:
         raise HTTPException(502, str(exc))