feat: surface inventory timestamps on nodes

Add creation_date, last_scan and last_modify to the node inventory and the
right-hand detail panel (last_seen already shown):

- creation_date  -> existing created_at column
- last_modify    -> existing updated_at column (bumped on any node change)
- last_scan      -> new column, stamped when a scan observes a node by IP/MAC
- last_seen      -> unchanged

Backend: new nodes.last_scan column + idempotent migration, NodeResponse
field, scanner stamps matching canvas nodes per scanned host. Frontend:
NodeData fields + DetailPanel rows with UTC-safe timestamp formatting.

ha-relevant: maybe

frontend/src/types/index.ts

@@ -102,6 +102,9 @@ export interface NodeData extends Record<string, unknown> {
   check_target?: string
   services: ServiceInfo[]
   last_seen?: string
+  last_scan?: string
+  created_at?: string
+  updated_at?: string
   response_time_ms?: number
   notes?: string
   cpu_count?: number