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/api/router.go

@@ -136,6 +136,7 @@ func (s *Server) Router() chi.Router {
 				r.Get("/", s.getProject)
 				r.Get("/stages/{stage}/env", s.listStageEnv)
 				r.Get("/stages/{stage}/instances", s.listInstances)
+				r.Get("/stages/{stage}/instances/{iid}/stats", s.getInstanceStats)
 				r.Get("/volumes", s.listVolumes)
 			})
 			r.Get("/deploys", s.listDeploys)