Refactor stats API for the new overview page with a user table.
This mainly includes:

• pagination for user stats queries
• separate endpoint for per-user device stats to be loaded once a row is expanded

Important: this includes integration tests for new API
Also look into changing response content to remove data manipulation from the frontend.

Result: VPN overview page on the dev instance loads user stats page by page as the table is scrolled. Device stats are fetched once a row is expanded.