feat: container monitoring backend core (PR-1)

- Add instance_events and health_checks tables with Alembic migration
- InstanceEventBus: typed pub/sub singleton with wildcard support
- HealthMonitor: async background loop polling containers every 15s
- SSE endpoint GET /events/stream with auth and connection limits
- Lifecycle hooks in tool_instances.py (create/start/stop/restart/delete)
- Structured JSON logging with correlation IDs
- 15 new unit tests (EventBus, HealthMonitor, MonitoringModels)

Quality gates: pytest 15 new passed, ruff clean

openspec/changes/container-monitoring-notifications/.openspec.yaml

@@ -0,0 +1,7 @@
+change: container-monitoring-notifications
+name: Container Monitoring and Notification System
+description: |
+  Monitor Docker container start, health, and lifecycle events with proper
+  logging and a real-time notification system for users.
+status: draft
+tasks: []