Ports xmrcheckout upstream features (7 commits since 655d037):
- SystemStatus model for reconciler heartbeat tracking
- Monero connectivity caching in system status
- Health status endpoints in routes
- Hardened reconciler status updates with SQLAlchemyError handling
- Status schema additions
Adapted for Wownero: 3-decimal precision (0.001 WOW) in amount rounding.
- POST/GET/DELETE /api/admin/users endpoints
- Admin UI at /admin with login, user list, create/delete
- Auth via ADMIN_API_KEY env var (separate from tenant auth)
- Tenants remain scoped to their own store only