Express — Google Calendar Test (Pro + Bay)
Endpoints:
  GET /health
  GET /auth
  GET /oauth2callback?code=...
  GET /freebusy?start=ISO&end=ISO
  GET /book?start=ISO&end=ISO&title=Lesson&desc=Intro&customer=user@example.com
  GET /list-bay?timeMin=ISO&timeMax=ISO
  GET /cancel/:eventId