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/pages/admin/UsersPage.tsx

@@ -1,5 +1,6 @@
 import { useState, useEffect, useCallback } from 'react'
-import { UserCheck, UserX, Shield, ArrowRightLeft } from 'lucide-react'
+import { useNavigate } from 'react-router-dom'
+import { UserCheck, UserX, Shield, ArrowRightLeft, ExternalLink } from 'lucide-react'
 import { DataTable, Pagination, SearchInput, PageHeader, StatusBadge, ActionMenu } from '@/components/admin'
 import type { Column } from '@/components/admin'
 import { Modal } from '@/components/common/Modal'
@@ -21,6 +22,7 @@ interface AdminUser {
 }
 
 export function UsersPage() {
+  const navigate = useNavigate()
   const [users, setUsers] = useState<AdminUser[]>([])
   const [loading, setLoading] = useState(true)
   const [search, setSearch] = useState('')
@@ -140,6 +142,11 @@ export function UsersPage() {
       className: 'w-12',
       render: (u) => (
         <ActionMenu items={[
+          {
+            label: 'View Detail',
+            icon: <ExternalLink className="h-4 w-4" />,
+            onClick: () => navigate(`/admin/users/${u.id}`),
+          },
           {
             label: 'Change Role',
             icon: <Shield className="h-4 w-4" />,