feat(observability): phase 8 - container stats, notifications & dashboard

Add container monitoring and notification system:
- Docker Stats API: real-time CPU/memory for running containers
- Webhook notifications for errors (deploy failures, stale, proxy unhealthy)
- Event log auto-pruning (daily, 30-day retention)
- ContainerStats component with auto-polling progress bars
- SystemHealthCard dashboard widget with running/proxy/error counts
- Full EN/RU i18n for stats and system health

internal/notify/types.go

@@ -0,0 +1,9 @@
+package notify
+
+// Event types for notifications.
+const (
+	EventTypeDeploySuccess = "deploy_success"
+	EventTypeDeployFailure = "deploy_failure"
+	EventTypeStaleDetected = "stale_detected"
+	EventTypeProxyUnhealthy = "proxy_unhealthy"
+)