EMX tracker

GET /api/v1/track/open/{token}.gif   open pixel
GET /api/v1/track/click/{token}      click redirect
GET /api/v1/unsubscribe/{token}      unsubscribe page
POST /api/v1/unsubscribe/{token}     one-click unsubscribe (RFC 8058)
GET /healthz                         health and event counters