feat: container monitoring integration + polish (PR-3)

- Integration tests: SSE auth, connection limits, lifecycle hooks, event persistence (6 tests)
- Instance events history API: GET /instances/{id}/events
- Documentation updates: terminal.md, backend.md, frontend.md
- Performance: SSE max 5 connections, health monitor write-on-change

Quality gates: pytest 21 monitoring passed, 172 unit passed (4 pre-existing), vitest 14 passed, tsc clean, eslint clean, ruff clean

docs/architecture/frontend.md

@@ -26,16 +26,21 @@ apps/web/src/
 │   ├── ssh_keys.ts      # SSH key API
 │   ├── tool_types.ts    # Tool type API
 │   ├── users.ts         # User API
+│   ├── events.ts        # SSE events API
 │   └── settings.ts      # Settings API
 ├── components/           # Reusable components
 │   ├── app-shell.tsx    # Main app layout
 │   ├── protected-route.tsx  # Auth guard
+│   ├── event-toast-bridge.tsx  # Events → toasts
 │   └── [more...]
-├── context/             # React contexts
-│   └── auth.tsx         # Auth state management
+├── state/               # Global state
+│   ├── auth.tsx         # Auth state management
+│   ├── events.tsx       # Event provider (SSE)
+│   └── toast.tsx        # Toast notifications
 ├── hooks/               # Custom hooks
 │   ├── use-auth.ts      # Auth hook
-│   └── use-theme.ts     # Theme hook
+│   ├── use-theme.ts     # Theme hook
+│   └── use-events.ts    # SSE events hook
 ├── pages/               # Page components (routes)
 │   ├── dashboard.tsx    # Dashboard
 │   ├── projects.tsx     # Project list
@@ -155,6 +160,28 @@ interface AuthState {
 }
 ```
 
+### Real-Time Events (SSE)
+
+The frontend receives real-time instance events via Server-Sent Events:
+
+```
+EventSource → useEvents() hook → EventProvider → EventToastBridge → ToastContainer
+```
+
+**Components:**
+- `useEvents()`: Manages SSE connection with auto-reconnect
+- `EventProvider`: Shares event stream across components
+- `EventToastBridge`: Maps events to toast notifications
+- `ToastContainer`: Displays and manages toast stack
+
+**Event-to-Toast Mapping:**
+| Event | Toast Severity | Auto-dismiss |
+|-------|---------------|--------------|
+| `instance.starting` | Info | 3s |
+| `instance.running` | Success | 3s |
+| `instance.error` | Error | Persistent |
+| `instance.stopped` | Info | 3s |
+
 ### 5. Routing Structure
 
 ```typescript
@@ -271,10 +298,10 @@ test('renders file list', () => {
 
 ## Future Improvements
 
+- [x] Implement real-time updates (SSE)
 - [ ] Add React Query for server state management
 - [ ] Implement virtual scrolling for large file trees
 - [ ] Add service worker for offline support
-- [ ] Implement real-time updates (WebSocket)
 - [ ] Add error boundary components
 
 ## Development Workflow