feat(admin): add OAuth sessions to MCP Access panel

Show active OAuth sessions (first) and static API tokens (second) in
the admin MCP Access tab. Admins can revoke any OAuth session, which
immediately terminates the live MCP transport for that client.

- Add admin-level listOAuthSessions / revokeOAuthSession in adminService
- Add GET /admin/oauth-sessions and DELETE /admin/oauth-sessions/:id routes
- Restructure AdminMcpTokensPanel into two sections; rename tab to MCP Access
- Fix stale writeAudit call in rotate-jwt-secret route (user_id → userId)
- Add admin.oauthSessions.* i18n keys across all 14 locale files

client/src/api/client.ts

@@ -232,6 +232,8 @@ export const adminApi = {
     apiClient.get('/admin/audit-log', { params }).then(r => r.data),
   mcpTokens: () => apiClient.get('/admin/mcp-tokens').then(r => r.data),
   deleteMcpToken: (id: number) => apiClient.delete(`/admin/mcp-tokens/${id}`).then(r => r.data),
+  oauthSessions: () => apiClient.get('/admin/oauth-sessions').then(r => r.data),
+  revokeOAuthSession: (id: number) => apiClient.delete(`/admin/oauth-sessions/${id}`).then(r => r.data),
   getPermissions: () => apiClient.get('/admin/permissions').then(r => r.data),
   updatePermissions: (permissions: Record<string, string>) => apiClient.put('/admin/permissions', { permissions }).then(r => r.data),
   rotateJwtSecret: () => apiClient.post('/admin/rotate-jwt-secret').then(r => r.data),