hub v0.110.0: allowlist + operator-only for offsite_proof_empty (R-87)

Controller v0.231.0 adds a nightly job that proves an app's newest off-site snapshot still
CONTAINS that app's data. When it finds one that does not, it emits offsite_proof_empty.

Two register lines, both load-bearing and both in this commit:
- allowedEventTypes - an unallowlisted type is answered 400 and VANISHES, so this entry is
  what makes the alarm exist at all.
- operatorOnlyEvents - a missing customerMessages entry is NOT a routing block
  (FormatCustomerEmail falls back to the raw message, the v0.78.0 defect that register was
  built for). A customer can take no action on a hollow recovery unit.

DELIBERATELY NOT reusing backup_integrity_failed, which is the nearest existing type: it
means THE STORE IS DAMAGED and carries the Hungarian template saying so. Here the store is
sound and the CONTENT is absent - different cause, different action, and telling a customer
their backups are damaged when they are not is the more expensive mistake. Same asymmetry
looksLikeRepositoryDamage is shaped around.

DELIBERATELY no customerMessages entry (the controller's dynamic Hungarian names the app and
what is missing; a template would discard it) and DELIBERATELY not in perAppCooldownEvents
(a fenced act - this job proves ONE app per night, so the coarse hourly cooldown is already
the right grain).

This widens the R-87 task's stated repo scope to felhom.eu/hub/. The reason is recorded in
felhom-controller/CONTEXT.md ruling 4 rather than left as an unexplained diff.

Hub green gate: go build/vet/test all pass, 18 packages.

hub/internal/notify/dispatcher.go

@@ -517,6 +517,16 @@ var operatorOnlyEvents = map[string]bool{
 	// block — `FormatCustomerEmail` falls back to the raw English message. That mistake shipped
 	// once (v0.78.0) and the comment above records it.
 	"backup_run_failures": true,
+	// R-87 (controller v0.231.0). The nightly off-site proof found a backup that is intact and holds
+	// none of the app's data. Operator-only for the same reason as `recovery_unit_capture_failed`
+	// above: the customer can take no action on it — a hollow recovery unit is a product/ops fault,
+	// diagnosed from the capture legs and the manifest, both of which are operator surfaces. The
+	// customer's actionable half of this class is the fill warning, which fires first.
+	//
+	// LISTED IN THE SAME COMMIT THAT MINTS THE TYPE, because a missing customerMessages entry is NOT
+	// a block — FormatCustomerEmail falls back to the raw message, which is the v0.78.0 defect this
+	// register was built for.
+	"offsite_proof_empty": true,
 	// R-197 (v0.93.0). "The sealed offsite repository key changed" is a custody fact about escrow
 	// blobs. A customer can take no action on it — the remedy is the operator's inspection of the
 	// off-site tier — and the text is operator-grade English naming host ids and retained-blob