feat(affiliate): add feature toggle and per-user custom invite settings

- 在系统设置「功能开关」中新增邀请返利总开关，默认关闭；
  关闭态：菜单隐藏、注册忽略 aff、新充值不返利，但已有 quota 仍可转余额
- 支持管理员为指定用户设置专属邀请码（覆盖随机码，全局唯一）
- 支持管理员为指定用户设置专属返利比例（覆盖全局比例，可单条/批量调整）
- 在系统设置邀请返利卡片内嵌入专属用户管理表格（搜索/编辑/批量/删除），
  删除采用项目通用 ConfirmDialog，会同时清除专属比例并把邀请码重置为系统随机码
- /affiliate 用户页新增「我的返利比例」卡片与动态使用说明，让用户直观看到
  分享后能拿到多少（同源 resolveRebateRatePercent 计算，与实际充值一致）
- 新增数据库迁移 132 添加 aff_rebate_rate_percent 与 aff_code_custom 列
- 新增 admin 路由组 /api/v1/admin/affiliates/users/* 共 5 个端点
- AffiliateService 改为只依赖 *SettingService，去除冗余的 SettingRepository
- 邀请码格式校验放宽到 [A-Z0-9_-]{4,32}，兼容旧 12 位系统码与新自定义码
- 补充单元测试与集成测试覆盖新方法、冲突路径与边界值

frontend/src/components/layout/AppSidebar.vue

@@ -634,6 +634,7 @@ const ChevronDownIcon = {
 const flagChannelMonitor = makeSidebarFlag(FeatureFlags.channelMonitor)
 const flagPayment = makeSidebarFlag(FeatureFlags.payment)
 const flagAvailableChannels = makeSidebarFlag(FeatureFlags.availableChannels)
+const flagAffiliate = makeSidebarFlag(FeatureFlags.affiliate)
 const flagOpsMonitoring = () => adminSettingsStore.opsMonitoringEnabled
 const flagAdminPayment = () => adminSettingsStore.paymentEnabled
 
@@ -656,7 +657,7 @@ function buildSelfNavItems(withDashboard: boolean): NavItem[] {
     { path: '/purchase', label: t('nav.buySubscription'), icon: RechargeSubscriptionIcon, hideInSimpleMode: true, featureFlag: flagPayment },
     { path: '/orders', label: t('nav.myOrders'), icon: OrderListIcon, hideInSimpleMode: true, featureFlag: flagPayment },
     { path: '/redeem', label: t('nav.redeem'), icon: GiftIcon, hideInSimpleMode: true },
-    { path: '/affiliate', label: t('nav.affiliate'), icon: UsersIcon, hideInSimpleMode: true },
+    { path: '/affiliate', label: t('nav.affiliate'), icon: UsersIcon, hideInSimpleMode: true, featureFlag: flagAffiliate },
     { path: '/profile', label: t('nav.profile'), icon: UserIcon },
     ...customMenuItemsForUser.value.map((item): NavItem => ({
       path: `/custom/${item.id}`,