hub v0.25.0: per-storage worst-fill alerting (StorageFillChecker)

Generalizes host_disk to any reported storage target (dump/backup volume, data drive,
thin pool, PBS). Per-(host,target) state, born/persistent, natural critical severity,
distinct storage_fill_* events; excludes the root-backed builtin (host_disk owns root).

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

@@ -1098,6 +1098,8 @@ var allowedEventTypes = map[string]bool{
 	// 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,
+	"storage_fill_warning":   true,
+	"storage_fill_critical":  true,
 	"expected_backup_missed": true,
 	"expected_dbdump_missed": true,
 	// Special