Handle Google token failures and report Google Calendar status (Feature 16) (#128)

Co-authored-by: orchestrator-agent <20+orchestrator-agent@code.cubecraftcreations.com>

internal/gcal/export_test.go

@@ -4,3 +4,7 @@ import "time"
 
 // SetClock replaces s's clock.
 func SetClock(s *Service, now func() time.Time) { s.now = now }
+
+// SetRetryDelay replaces s's wait before the first retry of a transient
+// Google failure.
+func SetRetryDelay(s *Service, d time.Duration) { s.retryDelay = d }