feat: watcher/CC system, SLA engine, and rich text comments

- Watcher system: ticket_watchers table, watch/unwatch endpoints,
  notifications to watchers on comments and updates, watcher/cc
  recipient sources in SendEmail scrip action, watch toggle and
  watcher avatars in ticket detail UI
- SLA engine: sla_policies table, SLA deadline columns on tickets,
  CRUD routes, OnSlaBreach scrip condition, scheduler SLA calculation,
  deadlines set on create/reply, cleared on resolve, SLA indicators
  on ticket list and detail, SLA Policies tab in admin
- Rich text: marked-based markdown rendering with XSS safety,
  Write/Preview toggle in comment composer, styled prose output

web/package.json

@@ -17,6 +17,7 @@
     "clsx": "^2.1.1",
     "date-fns": "^4.4.0",
     "lucide-react": "^1.17.0",
+    "marked": "^18.0.5",
     "next": "16.2.7",
     "next-themes": "^0.4.6",
     "react": "19.2.4",