Placement hardening (F-3a-1..4) + enlarge-blocked delivery chain (Task 3a-fix, v0.134.1)

PlaceOffsiteRestore: live target via raw GetStackHDDPath not AppNamespaceRoot (F-3a-1a: no SSD
merge; undeployed refused), placement headroom gate (F-3a-1b), stat pre-pass over all placements
before any copy (F-3a-4: no partial writes), scratch removed on success/kept on failure (F-3a-2).
mapOffsiteRestorePaths refuses the namespace root itself (F-3a-3).
Delivery chain: DefaultEnabledEvents + GetNotificationPrefs append-if-absent migration + settings
checkbox + handler slice; paired with hub v0.55.0 allowlist (no customerMessages entry — raw
dynamic message survives). +8 tests; all 6 controller §10 red-proofs verified.

controller/internal/web/templates/settings_notifications.html

@@ -38,6 +38,10 @@
                         <input type="checkbox" name="event_crossdrive_failed" {{with .NotificationPrefs}}{{range .EnabledEvents}}{{if eq . "crossdrive_failed"}}checked{{end}}{{end}}{{end}}>
                         <span class="toggle-label">Másodlagos mentés sikertelen</span>
                     </label>
+                    <label class="toggle">
+                        <input type="checkbox" name="event_offbox_enlarge_blocked" {{with .NotificationPrefs}}{{range .EnabledEvents}}{{if eq . "offbox_enlarge_blocked"}}checked{{end}}{{end}}{{end}}>
+                        <span class="toggle-label">Távoli mentés — tárhelykeret-figyelmeztetés</span>
+                    </label>
                     <label class="toggle">
                         <input type="checkbox" name="event_disk_alerts" {{with .NotificationPrefs}}{{range .EnabledEvents}}{{if eq . "disk_warning"}}checked{{end}}{{end}}{{end}}>
                         <span class="toggle-label">Lemez figyelmeztetés (90%+)</span>