feat: admin invite codes with plan assignment + user detail page

- Migration 030: add email, assigned_plan, trial_duration_days, email_sent_at
  to invite_codes with CHECK constraints
- Resend email integration (graceful degradation when API key not set)
- Invite codes now support plan assignment (free/pro/team) and trial duration (1-90 days)
- Registration applies invite code plan/trial to new subscription
- Auto-downgrade expired trials on authenticated access
- Enriched GET /admin/users/{id} with account, subscription, sessions, audit logs
- New endpoints: PUT /admin/users/{id}/subscription/plan and extend-trial
- Frontend: enhanced invite codes page with email, plan, trial fields
- Frontend: new user detail page at /admin/users/:userId
- Fixed API path drift: /invite-codes -> /invites
- 11 new backend tests, 416 total passing

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

frontend/src/router.tsx

@@ -21,6 +21,7 @@ const AccountSettingsPage = lazy(() => import('@/pages/AccountSettingsPage'))
 const AdminLayout = lazy(() => import('@/components/admin/AdminLayout'))
 const AdminDashboardPage = lazy(() => import('@/pages/admin/DashboardPage'))
 const AdminUsersPage = lazy(() => import('@/pages/admin/UsersPage'))
+const AdminUserDetailPage = lazy(() => import('@/pages/admin/UserDetailPage'))
 const AdminInviteCodesPage = lazy(() => import('@/pages/admin/InviteCodesPage'))
 const AdminAuditLogsPage = lazy(() => import('@/pages/admin/AuditLogsPage'))
 const AdminPlanLimitsPage = lazy(() => import('@/pages/admin/PlanLimitsPage'))
@@ -143,6 +144,14 @@ export const router = createBrowserRouter([
               </Suspense>
             ),
           },
+          {
+            path: 'users/:userId',
+            element: (
+              <Suspense fallback={<PageLoader />}>
+                <AdminUserDetailPage />
+              </Suspense>
+            ),
+          },
           {
             path: 'invite-codes',
             element: (