feat: access request UI — catalog badges, request modal, admin approval page
Backend:
- access_requests table in DuckDB schema
- AccessRequestRepository with create/approve/deny/list
- API: POST/GET /api/access-requests (submit, my requests, pending, approve, deny)
UI:
- Catalog: lock icon on private tables, "Request Access" button + modal
- Catalog: "Pending" badge for tables with pending requests
- Admin permissions page (/admin/permissions): approve/deny requests,
grant/revoke permissions, view all user permissions
- Cross-navigation between admin/tables and admin/permissions
733 tests passing.