feat: Hub monitoring takeover — event push system + config cleanup (v0.21.0)

Replace external Healthchecks.io with Hub-native event system. Controller
now pushes structured events via POST /api/v1/event with typed detail
structs. Hub handles dead man's switch, notification dispatch, and cooldowns.

Phase 5: PushEvent() core method, 21 event types, expanded notification
settings (11 toggles), Hub connection monitoring on dashboard, alerts.
Phase 6: Deprecation log for ping UUIDs, pinger kept for transition.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/settings/settings.go

@@ -85,11 +85,15 @@ type NotificationPrefs struct {
 
 // DefaultEnabledEvents are the events enabled by default for new customers.
 var DefaultEnabledEvents = []string{
-	"disk_warning",
 	"backup_failed",
-	"update_available",
+	"db_dump_failed",
+	"disk_warning",
+	"disk_critical",
 	"storage_disconnected",
-	"storage_reconnected",
+	"node_down",
+	"health_critical",
+	"expected_backup_missed",
+	"expected_dbdump_missed",
 }
 
 // DBValidationCache holds cached DB dump validation results.