Ticket IDs, audit log, markdown comments, tabbed detail page

- Tickets get a random display ID (V + 9 digits, e.g. V325813929)
- Ticket detail page has Overview / Comments / Audit Log tabs
- Audit log records every action (create, status, assignee, severity,
  reroute, title/overview edit, comment add/delete) with who and when
- Comments redesigned: avatar (initials + color), markdown rendering
  via react-markdown + remark-gfm, Write/Preview toggle
- Dashboard shows displayId and assignee avatar
- URLs now use displayId (/tickets/V325813929)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/pages/NewTicket.tsx

@@ -49,7 +49,7 @@ export default function NewTicket() {
       if (form.assigneeId) payload.assigneeId = form.assigneeId
 
       const res = await api.post('/tickets', payload)
-      navigate(`/tickets/${res.data.id}`)
+      navigate(`/tickets/${res.data.displayId}`)
     } catch {
       setError('Failed to create ticket')
     } finally {