feat(admin): Add email search and rate limit filtering for accounts and redeem codes

- Add used_by_email column to redeem code export CSV for better user identification
- Implement rate_limited status filter in account listing with RateLimitResetAt check
- Extend redeem code search to include user email in addition to code matching
- Add API key search capability to user listing filters
- Display user email in redeem code table used_by column for improved visibility
- Update search placeholders in UI to reflect expanded search capabilities (email, username, notes, API key)
- Improve Chinese and English localization strings for search hints

frontend/src/i18n/locales/en.ts

@@ -841,7 +841,7 @@ export default {
       createUser: 'Create User',
       editUser: 'Edit User',
       deleteUser: 'Delete User',
-      searchUsers: 'Search users...',
+      searchUsers: 'Search by email, username, notes, or API key...',
       allRoles: 'All Roles',
       allStatus: 'All Status',
       admin: 'Admin',
@@ -2129,7 +2129,7 @@ export default {
       title: 'Redeem Code Management',
       description: 'Generate and manage redeem codes',
       generateCodes: 'Generate Codes',
-      searchCodes: 'Search codes...',
+      searchCodes: 'Search codes or email...',
       allTypes: 'All Types',
       allStatus: 'All Status',
       balance: 'Balance',

frontend/src/i18n/locales/zh.ts

@@ -865,8 +865,8 @@ export default {
       editUser: '编辑用户',
       deleteUser: '删除用户',
       deleteConfirmMessage: "确定要删除用户 '{email}' 吗？此操作无法撤销。",
-      searchPlaceholder: '搜索用户邮箱或用户名、备注、支持模糊查询...',
-      searchUsers: '搜索用户邮箱或用户名、备注、支持模糊查询',
+      searchPlaceholder: '邮箱/用户名/备注/API Key 模糊搜索...',
+      searchUsers: '邮箱/用户名/备注/API Key 模糊搜索',
       roleFilter: '角色筛选',
       allRoles: '全部角色',
       allStatus: '全部状态',
@@ -2292,7 +2292,7 @@ export default {
       allStatus: '全部状态',
       unused: '未使用',
       used: '已使用',
-      searchCodes: '搜索兑换码...',
+      searchCodes: '搜索兑换码或邮箱...',
       exportCsv: '导出 CSV',
       deleteAllUnused: '删除全部未使用',
       deleteCodeConfirm: '确定要删除此兑换码吗？此操作无法撤销。',