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/types/admin.ts

@@ -128,3 +128,89 @@ export interface GlobalCategoryCreate {
   slug: string
   description?: string | null
 }
+
+// Invite code types (enhanced)
+export interface InviteCodeResponse {
+  id: string
+  code: string
+  created_by_id: string
+  used_by_id: string | null
+  expires_at: string | null
+  note: string | null
+  created_at: string
+  used_at: string | null
+  is_used: boolean
+  is_expired: boolean
+  is_valid: boolean
+  email: string | null
+  assigned_plan: string
+  trial_duration_days: number | null
+  email_sent_at: string | null
+  has_trial: boolean
+  email_sent: boolean
+}
+
+export interface InviteCodeCreateRequest {
+  expires_at?: string | null
+  note?: string | null
+  email?: string | null
+  assigned_plan?: 'free' | 'pro' | 'team'
+  trial_duration_days?: number | null
+}
+
+// User detail types
+export interface AccountSummary {
+  id: string
+  name: string
+  display_code: string | null
+}
+
+export interface SubscriptionSummary {
+  id: string
+  plan: string
+  status: string
+  current_period_start: string | null
+  current_period_end: string | null
+}
+
+export interface SessionSummary {
+  id: string
+  tree_name: string | null
+  started_at: string
+  completed_at: string | null
+  outcome: string | null
+}
+
+export interface AuditLogSummary {
+  id: string
+  action: string
+  resource_type: string | null
+  resource_id: string | null
+  created_at: string
+  details: Record<string, unknown> | null
+}
+
+export interface InviteCodeUsedSummary {
+  code: string
+  assigned_plan: string
+  trial_duration_days: number | null
+  created_by_email: string | null
+}
+
+export interface UserDetailResponse {
+  id: string
+  email: string
+  full_name: string | null
+  role: string
+  is_active: boolean
+  is_super_admin: boolean
+  is_team_admin: boolean
+  created_at: string
+  account: AccountSummary | null
+  subscription: SubscriptionSummary | null
+  invite_code_used: InviteCodeUsedSummary | null
+  recent_sessions: SessionSummary[]
+  total_sessions: number
+  recent_audit_logs: AuditLogSummary[]
+  total_audit_logs: number
+}