hub v0.23.0: host root-disk pressure monitoring + alert

New HostDiskChecker on the 60s sweep alerts the operator when a Proxmox host root
filesystem crosses warn (90%) / crit (95%). Born/persistent (a disk already full at
hub restart alerts on cycle 1); distinct host_disk_* event types from the guest disk_*;
critical band maps to severity error (the dispatcher only routes warning/error).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HxLA1mZurFq9kt8hneFeCs

hub/internal/api/handler.go

@@ -1095,6 +1095,9 @@ var allowedEventTypes = map[string]bool{
 	"host_stale":             true,
 	"host_down":              true,
 	"host_recovered":         true,
+	// Hub-generated host root-fs disk-pressure (v0.23.0) — distinct from the controller's GUEST disk_*
+	"host_disk_warning":      true,
+	"host_disk_critical":     true,
 	"expected_backup_missed": true,
 	"expected_dbdump_missed": true,
 	// Special