hub v0.81.0 — E-2: backup_target_absent gets its own signal (ships first)

An event type the hub does not allowlist makes POST /event return 400 and the
event vanishes (R-97a). The controller cannot emit backup_target_absent until
this is live, so the hub half ships first.

E-2 Phase 0 established that an absent backup target has NO prompt signal today.
The controller's drive-gate path stops apps and logs a WARN but emits nothing:
NotifyStorageDisconnected is defined and never called anywhere (verified against
the gitignored-cmd/ trap with a positive control). A drive that is ONLY a backup
target has no apps to stop, so it is entirely silent. The sole signal is the
tier's own failure at its next due cycle -- up to ~24h on the daily local tier,
which is the R-100 shape: a real fault visible only after a deadline elapses.

Added to BOTH registers, because each half fails differently:
  allowedEventTypes  -- without it the event is lost at the door;
  customerMessages   -- without it the event IS delivered but in the controller's
                        raw operator English, and nothing looks broken.

backup_target_absent is deliberately NOT folded into storage_disconnected: that
says "a drive went away and some apps may have stopped"; this says "the thing
that makes your backup survive a disk failure is gone".

Hungarian copy names the consequence, not just the fact. backup_target_restored
is the paired recovery at info severity -- severityNotifies NOT widened.

Three tests pin the pair and the copy's substance. All red-proofed with the
mutation VERIFIED to have landed first: the initial attempt silently no-op'd
(gofmt had realigned the map) and the test "passed" -- a false proof that would
have been reported as evidence.

Green gate: build + vet + test rc=0, run separately from this commit.

hub/internal/api/handler.go

@@ -1595,8 +1595,16 @@ var allowedEventTypes = map[string]bool{
 	"offbox_repo_reset":    true,
 	"storage_disconnected": true,
 	"storage_reconnected":  true,
-	"disk_warning":         true,
-	"disk_critical":        true,
+	// controller v0.184.0 (E-2) — the assigned whole-guest backup TARGET drive is absent. Distinct
+	// from storage_disconnected on purpose: that one says "a drive went away and some apps may have
+	// stopped"; this one says "the thing that makes a backup survive a disk failure is gone", which
+	// is a different action for the customer and a different urgency for the operator. Before this
+	// the only signal was the tier's own failure at its next due cycle — up to ~24 h on the daily
+	// local tier — i.e. the R-100 shape: a real fault visible only after a deadline elapsed.
+	"backup_target_absent":   true,
+	"backup_target_restored": true,
+	"disk_warning":           true,
+	"disk_critical":          true,
 	// controller v0.169.0 — per-disk SMART degradation (Rendben→Figyelmeztetés/Hiba). The controller
 	// sends a dynamic Hungarian message (disk label + the triggering attribute names), so — like
 	// offbox_enlarge_blocked — there is deliberately NO customerMessages entry (which would discard the