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/backend.md

@@ -17,6 +17,10 @@ The Headquarter backend is built with **FastAPI** and follows a layered architec
 │  │  Auth   │ │ Projects │ │ Users  │ │   Git    │          │
 │  │ Routes  │ │ Routes   │ │ Routes │ │ Repos    │          │
 │  └────┬────┘ └────┬─────┘ └───┬────┘ └────┬─────┘          │
+│  ┌──────────┐ ┌──────────┐                                  │
+│  │ ToolInst │ │  Events  │                                  │
+│  │ Routes   │ │ Routes   │                                  │
+│  └────┬─────┘ └────┬─────┘                                  │
 ├───────┼───────────┼───────────┼───────────┼─────────────────┤
 │       │           │           │           │                  │
 │  Auth │  Project  │   User    │   Git     │                  │
@@ -41,7 +45,9 @@ src/
 │   ├── git_repositories.py # Repository endpoints
 │   ├── users.py           # User endpoints
 │   ├── tool_types.py      # Tool type endpoints
+│   ├── tool_instances.py  # Tool instance endpoints
 │   ├── ssh_keys.py        # SSH key endpoints
+│   ├── events.py          # SSE streaming endpoint
 │   └── dashboard.py       # Dashboard endpoints
 ├── auth/                   # Authentication
 │   ├── session.py         # Session management
@@ -54,7 +60,15 @@ src/
 │   ├── git_repository.py  # Repository model
 │   ├── tool_type.py       # Tool type model
 │   ├── ssh_key.py         # SSH key model
+│   ├── instance_event.py  # Instance event audit model
+│   ├── health_check.py    # Health check snapshot model
 │   └── user_config.py     # User config model
+├── services/               # Services
+│   ├── docker.py          # Docker operations
+│   ├── terminal_manager.py # Terminal session manager
+│   ├── event_bus.py       # Instance event bus (pub/sub)
+│   ├── health_monitor.py  # Background health monitoring
+│   └── lifecycle_hooks.py # Instance lifecycle events
 ├── utils/                  # Utilities
 │   ├── git_url_parser.py  # URL parsing
 │   ├── git_files.py       # Git file operations
@@ -193,6 +207,33 @@ Errors are handled at multiple levels:
 - **Integration tests**: PostgreSQL with transaction rollback
 - **Fixtures**: Shared in `conftest.py`
 
+## Monitoring & Notifications
+
+The backend includes a real-time monitoring system:
+
+### Components
+
+- **InstanceEventBus** (`services/event_bus.py`): Typed pub/sub singleton for instance lifecycle events
+- **HealthMonitor** (`services/health_monitor.py`): Asyncio background task polling container health every 15s
+- **SSE Endpoint** (`api/events.py`): Server-Sent Events streaming for real-time frontend updates
+- **Lifecycle Hooks** (`services/lifecycle_hooks.py`): Publishes events on create/start/stop/restart/delete
+
+### Event Flow
+
+```
+Container Action → Lifecycle Hook → EventBus → SSE Stream → Frontend Toast
+```
+
+### Event Types
+
+| Event | When Fired |
+|-------|-----------|
+| `instance.created` | After DB insert |
+| `instance.starting` | Before docker compose up |
+| `instance.running` | After readiness probe succeeds |
+| `instance.error` | Build fail, crash, or probe fail |
+| `instance.stopped` | After docker compose stop |
+
 ## Technology Stack
 
 | Component | Technology | Version |