Lists: server-side row filtering, sorting and pagination #70

Gap

GET /api/users/{username}/lists/{id}/data is documented as "list data rows for a public list with pagination, filtering, sorting". GET /api/lists/{id}/data takes the same shape. ViewModels/ListsViewModel.cs fetches rows without using any of it, so large lists are fetched wholesale and cannot be filtered or sorted server-side.

Acceptance criteria

• Confirm the supported query parameters against the live OpenAPI spec and a real request.
• GetListDataAsync accepts sort, filter and paging arguments.
• Grid column headers sort server-side; a filter row filters server-side.
• Paging uses the cursor if the endpoint offers one (consistent with the feed work).
• Filters/sorts are what a saved view's config persists — coordinate with the saved-views issue.