feat: Gateway health monitoring (Hermes CLI, API, frontend)

- Hermes CLI wrapper using subprocess.run for gateway status
- Gateway API: status list (with filter), events (placeholder), start/stop/restart with audit logging
- Frontend GatewayPage: timeline, filter bar, status table with 10s polling, operation modals
- usePolling hook for periodic data refresh

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

frontend/src/api/gateway.ts

@@ -0,0 +1,46 @@
+import client from './client'
+
+export interface GatewayInfo {
+  id: string
+  name: string
+  status: 'online' | 'offline' | 'warn'
+  downtime_minutes: number
+  restarts_today: number
+  last_event: string
+  profile_type: string
+}
+
+export interface GatewayEvent {
+  time: string
+  profile: string
+  type: string
+  detail: string
+}
+
+export interface OperationResult {
+  success: boolean
+  action: string
+  profile: string
+  detail: string
+}
+
+export function getGatewayStatus(status?: string) {
+  const params = status ? { status } : undefined
+  return client.get<GatewayInfo[]>('/gateway/status', { params })
+}
+
+export function getGatewayEvents() {
+  return client.get<GatewayEvent[]>('/gateway/events')
+}
+
+export function startGateway(profile: string) {
+  return client.post<OperationResult>(`/gateway/${profile}/start`)
+}
+
+export function stopGateway(profile: string) {
+  return client.post<OperationResult>(`/gateway/${profile}/stop`)
+}
+
+export function restartGateway(profile: string) {
+  return client.post<OperationResult>(`/gateway/${profile}/restart`)
+}