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/features/terminal.md

@@ -50,6 +50,23 @@ Standard terminal shortcuts work as expected:
 
 Special keys can be accessed via the special keys panel on mobile or by using modifier combinations.
 
+## Container Monitoring & Notifications
+
+The platform monitors your tool instances in real-time and notifies you of important events:
+
+### What You'll See
+
+- **Starting:** When a container begins starting
+- **Running:** When a container is ready
+- **Error:** When a build fails, container crashes, or tunnel fails
+- **Stopped:** When a container stops
+
+Notifications appear as toast messages at the top of the screen. Errors persist until dismissed; other notifications auto-dismiss after a few seconds.
+
+### Real-Time Status
+
+Instance status badges update in real-time via Server-Sent Events (SSE) — no page refresh needed.
+
 ## Troubleshooting
 
 ### Connection Issues
@@ -59,6 +76,10 @@ Special keys can be accessed via the special keys panel on mobile or by using mo
 - Network issues - the client will auto-reconnect
 - Session timeout - sessions expire after 30 minutes of inactivity
 
+**"Container not found" error (4004):**
+- The Docker container no longer exists (e.g., after host restart)
+- Restart the tool instance to recreate the container
+
 **Terminal not responding:**
 - Try resetting the terminal using the Reset button
 - Check if the tool instance is still running