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/types/index.ts

@@ -39,8 +39,19 @@ export interface Comment {
   createdAt: string
 }
 
+export interface AuditLog {
+  id: string
+  ticketId: string
+  userId: string
+  action: string
+  detail: string | null
+  createdAt: string
+  user: Pick<User, 'id' | 'username' | 'displayName'>
+}
+
 export interface Ticket {
   id: string
+  displayId: string
   title: string
   overview: string
   severity: number