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/components/panels/DetailPanel.tsx

@@ -14,6 +14,13 @@ interface DetailPanelProps {
   onEdit: (id: string) => void
 }
 
+// Backend timestamps may arrive without a timezone suffix (naive UTC). Append
+// 'Z' when absent so the browser parses them as UTC rather than local time.
+function formatTimestamp(value: string): string {
+  const iso = /[Zz]|[+-]\d{2}:?\d{2}$/.test(value) ? value : value + 'Z'
+  return new Date(iso).toLocaleString()
+}
+
 type SvcForm = { port: string; protocol: 'tcp' | 'udp'; service_name: string; path: string }
 const EMPTY_FORM: SvcForm = { port: '', protocol: 'tcp', service_name: '', path: '' }
 
@@ -252,7 +259,10 @@ export function DetailPanel({ onEdit }: DetailPanelProps) {
         {data.mac && <DetailRow label="MAC" value={data.mac} mono />}
         {data.os && <DetailRow label="OS" value={data.os} />}
         {data.check_method && <DetailRow label="Check" value={data.check_method} mono />}
-        {data.last_seen && <DetailRow label="Last Seen" value={new Date(/[Zz]|[+-]\d{2}:?\d{2}$/.test(data.last_seen) ? data.last_seen : data.last_seen + 'Z').toLocaleString()} />}
+        {data.last_seen && <DetailRow label="Last Seen" value={formatTimestamp(data.last_seen)} />}
+        {data.last_scan && <DetailRow label="Last Scan" value={formatTimestamp(data.last_scan)} />}
+        {data.created_at && <DetailRow label="Created" value={formatTimestamp(data.created_at)} />}
+        {data.updated_at && <DetailRow label="Last Modified" value={formatTimestamp(data.updated_at)} />}
       </div>
 
       {/* Size section — manual width/height entry for pixel-exact sizing */}

frontend/src/components/panels/__tests__/DetailPanel.test.tsx

@@ -212,6 +212,37 @@ describe('DetailPanel', () => {
     })
   })
 
+  describe('Inventory timestamps', () => {
+    it('renders Created, Last Scan and Last Modified rows when present', () => {
+      setupStore({
+        created_at: '2026-01-02T10:00:00Z',
+        last_scan: '2026-06-01T08:30:00Z',
+        updated_at: '2026-06-20T12:00:00Z',
+        last_seen: '2026-06-25T09:15:00Z',
+      })
+      render(<DetailPanel onEdit={vi.fn()} />)
+      expect(screen.getByText('Created')).toBeInTheDocument()
+      expect(screen.getByText('Last Scan')).toBeInTheDocument()
+      expect(screen.getByText('Last Modified')).toBeInTheDocument()
+      expect(screen.getByText('Last Seen')).toBeInTheDocument()
+    })
+
+    it('omits rows whose timestamps are absent', () => {
+      setupStore({ created_at: '2026-01-02T10:00:00Z' })
+      render(<DetailPanel onEdit={vi.fn()} />)
+      expect(screen.getByText('Created')).toBeInTheDocument()
+      expect(screen.queryByText('Last Scan')).not.toBeInTheDocument()
+      expect(screen.queryByText('Last Modified')).not.toBeInTheDocument()
+    })
+
+    it('parses naive (suffix-less) UTC timestamps without throwing', () => {
+      setupStore({ created_at: '2026-01-02 10:00:00' })
+      render(<DetailPanel onEdit={vi.fn()} />)
+      // Renders a locale string for the date (year is locale-independent).
+      expect(screen.getByText(/2026/)).toBeInTheDocument()
+    })
+  })
+
   describe('Panel actions', () => {
     it('calls setSelectedNode(null) when close button is clicked', () => {
       const setSelectedNode = vi.fn()

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