🎫 feat: Enhance redemption code expiry handling & improve UI responsiveness

Backend
• Introduced `validateExpiredTime` helper in `controller/redemption.go`; reused in both Add & Update endpoints to enforce “expiry time must not be earlier than now”, eliminating duplicated checks
• Removed `RedemptionCodeStatusExpired` constant and all related references – expiry is now determined exclusively by the `expired_time` field for simpler, safer state management
• Simplified `DeleteInvalidRedemptions`: deletes codes that are `used` / `disabled` or `enabled` but already expired, without relying on extra status codes
• Controller no longer mutates `status` when listing or fetching redemption codes; clients derive expiry status from timestamp

Frontend
• Added reusable `isExpired` helper in `RedemptionsTable.js`; leveraged for:
  – status rendering (orange “Expired” tag)
  – action-menu enable/disable logic
  – row styling
• Removed duplicated inline expiry logic, improving readability and performance
• Adjusted toolbar layout: on small screens the “Clear invalid codes” button now wraps onto its own line, while “Add” & “Copy” remain grouped

Result
The codebase is now more maintainable, secure, and performant with no redundant constants, centralized validation, and cleaner UI behaviour across devices.

web/src/i18n/locales/en.json

@@ -1655,5 +1655,9 @@
   "设置保存失败": "Settings save failed",
   "已新增 {{count}} 个模型：{{list}}": "Added {{count}} models: {{list}}",
   "未发现新增模型": "No new models were added",
-  "令牌用于API访问认证，可以设置额度限制和模型权限。": "Tokens are used for API access authentication, and can set quota limits and model permissions."
+  "令牌用于API访问认证，可以设置额度限制和模型权限。": "Tokens are used for API access authentication, and can set quota limits and model permissions.",
+  "清除失效兑换码": "Clear invalid redemption codes",
+  "确定清除所有失效兑换码？": "Are you sure you want to clear all invalid redemption codes?",
+  "将删除已使用、已禁用及过期的兑换码，此操作不可撤销。": "This will delete all used, disabled, and expired redemption codes, this operation cannot be undone.",
+  "选择过期时间（可选，留空为永久）": "Select expiration time (optional, leave blank for permanent)"
 }