feat: 用户列表显示订阅分组及剩余天数

- User模型新增Subscriptions关联
- 用户列表批量加载订阅信息避免N+1查询
- GroupBadge组件支持显示剩余天数（过期红色、<=3天红色、<=7天橙色）
- 用户管理页面新增订阅分组列

frontend/src/i18n/locales/en.ts

@@ -449,6 +449,7 @@ export default {
       columns: {
         user: 'User',
         role: 'Role',
+        subscriptions: 'Subscriptions',
         balance: 'Balance',
         usage: 'Usage',
         concurrency: 'Concurrency',
@@ -458,6 +459,9 @@ export default {
       },
       today: 'Today',
       total: 'Total',
+      noSubscription: 'No subscription',
+      daysRemaining: '{days}d',
+      expired: 'Expired',
       disableUser: 'Disable User',
       enableUser: 'Enable User',
       viewApiKeys: 'View API Keys',

frontend/src/i18n/locales/zh.ts

@@ -467,15 +467,20 @@ export default {
       columns: {
         email: '邮箱',
         role: '角色',
+        subscriptions: '订阅分组',
         balance: '余额',
         usage: '用量',
         concurrency: '并发数',
         status: '状态',
         created: '创建时间',
         actions: '操作',
+        user: '用户',
       },
       today: '今日',
       total: '累计',
+      noSubscription: '暂无订阅',
+      daysRemaining: '{days}天',
+      expired: '已过期',
       roles: {
         admin: '管理员',
         user: '用户',