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/api.ts

@@ -1,5 +1,6 @@
 import type {
 	ApiEnvelope,
+	ContainerStats,
 	Deploy,
 	DeployLog,
 	EventLogEntry,
@@ -420,4 +421,16 @@ export function bulkCleanupStaleContainers(): Promise<{ deleted: number }> {
 	return post<{ deleted: number }>('/api/containers/stale/cleanup');
 }
 
+// ── Container Stats ────────────────────────────────────────────────
+
+export function fetchContainerStats(
+	projectId: string,
+	stageId: string,
+	instanceId: string
+): Promise<ContainerStats> {
+	return get<ContainerStats>(
+		`/api/projects/${projectId}/stages/${stageId}/instances/${instanceId}/stats`
+	);
+}
+
 export { ApiError };