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/views/admin/RedeemView.vue

@@ -117,9 +117,9 @@
             </span>
           </template>
 
-          <template #cell-used_by="{ value }">
+          <template #cell-used_by="{ value, row }">
             <span class="text-sm text-gray-500 dark:text-dark-400">
-              {{ value ? t('admin.redeem.userPrefix', { id: value }) : '-' }}
+              {{ row.user?.email || (value ? t('admin.redeem.userPrefix', { id: value }) : '-') }}
             </span>
           </template>