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/package.json

@@ -16,7 +16,9 @@
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-hook-form": "^7.54.2",
+    "react-markdown": "^9.0.1",
     "react-router-dom": "^6.28.0",
+    "remark-gfm": "^4.0.0",
     "zod": "^3.23.8"
   },
   "devDependencies": {