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

web/src/lib/i18n/en.json

@@ -504,6 +504,17 @@
     },
     "metadata": "Details"
   },
+  "stats": {
+    "cpu": "CPU",
+    "mem": "MEM",
+    "unavailable": "Stats unavailable"
+  },
+  "systemHealth": {
+    "title": "System Health",
+    "containers": "Containers",
+    "proxies": "Proxies",
+    "recentErrors": "Recent Errors"
+  },
   "language": {
     "en": "English",
     "ru": "Russian"