[Backend] API and support for web-admin dashboard

[MahadAhmed25]

Scope:
Add GET /admin/stats (or GET /stats) returning e.g. { userCount, eventCount, upcomingEventCount? } so the dashboard doesn’t need to fetch full lists for counts.
Ensure CORS and any env/docs (e.g. .env.example) are set so the web-admin can call the API.