v0.78.0 asserted in a comment that a type with no customerMessages entry cannot
reach a customer. It can: templates.go falls back to the raw message when the
entry is missing, and the only customer gate is prefs.EnabledEvents — pure
configuration. A customer with whole_guest_backup_failed enabled would have been
emailed raw English operator text about a backup they cannot act on. The new test
proves it against the v0.78.0 shape.
operatorOnlyEvents is now an explicit register checked before prefs, logging a
skipped/operator_only row so the skip is visible. NOT implemented as 'missing
customerMessages blocks delivery' — several types rely on that fallback on
purpose. The handler comment now names the real mechanism.
