feat: notification center backend integration (PR-2)

- Wire lifecycle_hooks.py to NotificationService after event bus publish
- Wire health_monitor.py to NotificationService after state changes
- Category/severity mapping: instance.* → info, error → error, unhealthy → warning
- Extend UserConfig API with notification_mute_categories and notification_toast_level
- 6 integration tests for event-to-notification flow
- All producer calls wrapped in try/except — failures logged, pipeline continues

Quality gates: pytest 41 passed (monitoring + lifecycle), ruff clean

apps/api/src/services/health_monitor.py

@@ -15,6 +15,7 @@ from src.models.tool_instance import ToolInstance
 from src.services.correlation import get_correlation_id
 from src.services.docker import check_tunnel_health, get_container_status
 from src.services.event_bus import InstanceEventBus, InstanceEventPayload
+from src.services.notification_service import notification_service
 
 logger = logging.getLogger(__name__)
 
@@ -217,3 +218,36 @@ class HealthMonitor:
         }
 
         await self._event_bus.publish(event_type, payload)
+
+        # Create notification for instance owner (fire-and-forget)
+        if new_status == "error":
+            category = "instance"
+            severity = "error"
+            title = "Container failed"
+        else:
+            category = "health"
+            if new_status == "unhealthy":
+                severity = "warning"
+                title = "Container unhealthy"
+            else:
+                severity = "info"
+                title = "Container recovered"
+
+        try:
+            await notification_service.create_notification(
+                session=session,
+                user_id=instance.owner_id,
+                category=category,
+                severity=severity,
+                title=title,
+                message=message,
+                source_type="tool_instances",
+                source_id=instance.id,
+                metadata=metadata,
+            )
+        except Exception:
+            logger.exception(
+                "Failed to create notification for health event %s",
+                event_type,
+                extra={"correlation_id": correlation_id},
+            )