feat: host inventory (agent.inventory), host detail page, UI columns

Store hardware/software snapshots on hosts; warn on hardware changes. Host card from Hosts list. Add agent version column to Events/Overview; rename Hosts table columns and sort by status.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/services/event_summary.py

@@ -10,6 +10,7 @@ def event_to_summary(event: Event) -> EventSummary:
         host_id=event.host_id,
         hostname=host.hostname,
         display_name=host.display_name,
+        product_version=host.product_version,
         occurred_at=event.occurred_at,
         received_at=event.received_at,
         category=event.category,