feat(admin): web admin panel — user CRUD, role assignment, system health (#125)

- Add BetterAuth admin plugin with admin/member role configuration
- Add banned/banReason/banExpires fields to users schema
- Gateway AdminModule: user list/create/role/ban/delete endpoints at /api/admin/users
- Gateway AdminHealthController: system health at /api/admin/health (DB, Valkey, agent pool, providers)
- AdminGuard: enforces admin role on all /api/admin/* routes
- Web admin page: two-tab UI (User Management + System Health)
- AdminRoleGuard: client-side redirect for non-admin users
- Add adminClient to web auth-client for BetterAuth admin plugin integration
- Add @mosaic/queue as gateway dep for Valkey health check

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

pnpm-lock.yaml

@@ -74,6 +74,9 @@ importers:
       '@mosaic/memory':
         specifier: workspace:^
         version: link:../../packages/memory
+      '@mosaic/queue':
+        specifier: workspace:^
+        version: link:../../packages/queue
       '@mosaic/telegram-plugin':
         specifier: workspace:^
         version: link:../../plugins/telegram