refactor: split pagination logic from response models

- Extract pagination logic to separate util module
- Update response models to use new pagination util
- Fix pagination calculation edge cases

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/utils/pagination.py

@@ -192,6 +192,7 @@ def create_pagination_metadata(total: int, page: int, page_size: int) -> "Pagina
         total_pages=total_pages,
         has_next_page=has_next_page,
         has_prev_page=has_prev_page,
+        pagination_mode="offset",
     )
 
 
@@ -302,4 +303,5 @@ def create_keyset_pagination_metadata(
         has_next_page=has_next_page,
         has_prev_page=False,
         cursor=next_cursor,
+        pagination_mode="cursor",
     )