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

apps/api/src/services/correlation.py

@@ -0,0 +1,32 @@
+"""Async correlation ID context variable and helpers."""
+
+import contextvars
+import uuid
+
+from fastapi import Request
+from starlette.middleware.base import BaseHTTPMiddleware
+
+CORRELATION_ID: contextvars.ContextVar[str] = contextvars.ContextVar("correlation_id")
+
+
+def get_correlation_id() -> str:
+    """Return the current correlation ID or generate a new UUID."""
+    try:
+        return CORRELATION_ID.get()
+    except LookupError:
+        return str(uuid.uuid4())
+
+
+class CorrelationIdMiddleware(BaseHTTPMiddleware):
+    """Set correlation ID from X-Request-ID header or generate a new UUID."""
+
+    async def dispatch(self, request: Request, call_next):
+        request_id = request.headers.get("X-Request-ID")
+        correlation_id = request_id or str(uuid.uuid4())
+        token = CORRELATION_ID.set(correlation_id)
+        try:
+            response = await call_next(request)
+            response.headers["X-Request-ID"] = correlation_id
+            return response
+        finally:
+            CORRELATION_ID.reset(token)