feat: 添加用户余额充值/退款功能 (#17)

## 功能特性

### 前端
- 在用户列表操作列添加充值和退款按钮
- 实现充值/退款对话框，支持输入金额和备注
- 从编辑用户表单中移除余额字段，防止直接修改
- 添加余额不足验证，实时显示操作后余额
- 优化备注提示词，提供多种场景示例

### 后端
- 为 redeem_codes 表添加 notes 字段（迁移文件）
- 在 UpdateUserBalance 接口添加 notes 参数支持
- 添加余额验证：金额必须大于0，操作后余额不能为负
- UpdateUser 接口移除 balance 字段处理，防止误操作
- 完整的审计日志和缓存管理

## 安全保护

- 前端：余额不足时禁用提交按钮，实时提示
- 后端：双重验证（输入金额 > 0 + 结果余额 >= 0）
- 权限：仅管理员可访问（AdminAuth 中间件）
- 审计：所有操作记录到 redeem_codes 表

## 修改文件

后端：
- backend/migrations/004_add_redeem_code_notes.sql
- backend/internal/model/redeem_code.go
- backend/internal/service/admin_service.go
- backend/internal/handler/admin/user_handler.go

前端：
- frontend/src/views/admin/UsersView.vue
- frontend/src/api/admin/users.ts
- frontend/src/i18n/locales/zh.ts
- frontend/src/i18n/locales/en.ts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/src/i18n/locales/en.ts

@@ -520,6 +520,26 @@ export default {
       allowedGroupsUpdated: 'Allowed groups updated successfully',
       failedToLoadGroups: 'Failed to load groups',
       failedToUpdateAllowedGroups: 'Failed to update allowed groups',
+      deposit: 'Deposit',
+      withdraw: 'Withdraw',
+      depositAmount: 'Deposit Amount',
+      withdrawAmount: 'Withdraw Amount',
+      currentBalance: 'Current Balance',
+      depositNotesPlaceholder: 'e.g., New user registration bonus, promotional credit, compensation, etc.',
+      withdrawNotesPlaceholder: 'e.g., Service issue refund, incorrect charge reversal, account closure refund, etc.',
+      notesOptional: 'Notes are optional but helpful for record keeping',
+      amountHint: 'Please enter a positive amount',
+      newBalance: 'New Balance',
+      depositing: 'Depositing...',
+      withdrawing: 'Withdrawing...',
+      confirmDeposit: 'Confirm Deposit',
+      confirmWithdraw: 'Confirm Withdraw',
+      depositSuccess: 'Deposit successful',
+      withdrawSuccess: 'Withdraw successful',
+      failedToDeposit: 'Failed to deposit',
+      failedToWithdraw: 'Failed to withdraw',
+      useDepositWithdrawButtons: 'Please use deposit/withdraw buttons to adjust balance',
+      insufficientBalance: 'Insufficient balance, balance cannot be negative after withdrawal',
     },
 
     // Groups

frontend/src/i18n/locales/zh.ts

@@ -587,6 +587,25 @@ export default {
       allowedGroupsUpdated: '允许分组更新成功',
       failedToLoadGroups: '加载分组列表失败',
       failedToUpdateAllowedGroups: '更新允许分组失败',
+      deposit: '充值',
+      withdraw: '退款',
+      depositAmount: '充值金额',
+      withdrawAmount: '退款金额',
+      depositNotesPlaceholder: '例如：新用户注册奖励、活动充值、补偿充值等',
+      withdrawNotesPlaceholder: '例如：服务问题退款、错误充值退回、账户注销退款等',
+      notesOptional: '备注为可选项，有助于未来查账',
+      amountHint: '请输入正数金额',
+      newBalance: '操作后余额',
+      depositing: '充值中...',
+      withdrawing: '退款中...',
+      confirmDeposit: '确认充值',
+      confirmWithdraw: '确认退款',
+      depositSuccess: '充值成功',
+      withdrawSuccess: '退款成功',
+      failedToDeposit: '充值失败',
+      failedToWithdraw: '退款失败',
+      useDepositWithdrawButtons: '请使用充值/退款按钮调整余额',
+      insufficientBalance: '余额不足，退款后余额不能为负数',
     },
 
     // Groups Management