feat: 向用户显示管理员调整余额的备注

- 为RedeemCode DTO添加notes字段（仅用于admin_balance/admin_concurrency类型）
- 更新mapper使其有条件地包含备注信息
- 在用户兑换历史UI中显示备注
- 备注以斜体显示，悬停时显示完整内容

用户现在可以看到管理员调整其余额的原因说明。

Changes:
- backend/internal/handler/dto/types.go: RedeemCode添加notes字段
- backend/internal/handler/dto/mappers.go: 条件性填充notes
- frontend/src/api/redeem.ts: TypeScript接口添加notes
- frontend/src/views/user/RedeemView.vue: UI显示备注信息

backend/internal/handler/dto/mappers.go

@@ -321,7 +321,7 @@ func RedeemCodeFromServiceAdmin(rc *service.RedeemCode) *AdminRedeemCode {
 }
 
 func redeemCodeFromServiceBase(rc *service.RedeemCode) RedeemCode {
-	return RedeemCode{
+	out := RedeemCode{
 		ID:           rc.ID,
 		Code:         rc.Code,
 		Type:         rc.Type,
@@ -335,6 +335,14 @@ func redeemCodeFromServiceBase(rc *service.RedeemCode) RedeemCode {
 		User:         UserFromServiceShallow(rc.User),
 		Group:        GroupFromServiceShallow(rc.Group),
 	}
+
+	// For admin_balance/admin_concurrency types, include notes so users can see
+	// why they were charged or credited by admin
+	if (rc.Type == "admin_balance" || rc.Type == "admin_concurrency") && rc.Notes != "" {
+		out.Notes = &rc.Notes
+	}
+
+	return out
 }
 
 // AccountSummaryFromService returns a minimal AccountSummary for usage log display.

backend/internal/handler/dto/types.go

@@ -198,6 +198,10 @@ type RedeemCode struct {
 	GroupID      *int64 `json:"group_id"`
 	ValidityDays int    `json:"validity_days"`
 
+	// Notes is only populated for admin_balance/admin_concurrency types
+	// so users can see why they were charged or credited
+	Notes *string `json:"notes,omitempty"`
+
 	User  *User  `json:"user,omitempty"`
 	Group *Group `json:"group,omitempty"`
 }

frontend/src/api/redeem.ts

@@ -14,7 +14,9 @@ export interface RedeemHistoryItem {
   status: string
   used_at: string
   created_at: string
-  // 订阅类型专用字段
+  // Notes from admin for admin_balance/admin_concurrency types
+  notes?: string
+  // Subscription-specific fields
   group_id?: number
   validity_days?: number
   group?: {

frontend/src/views/user/RedeemView.vue

@@ -312,6 +312,14 @@
                 <p v-else class="text-xs text-gray-400 dark:text-dark-500">
                   {{ t('redeem.adminAdjustment') }}
                 </p>
+                <!-- Display notes for admin adjustments -->
+                <p
+                  v-if="item.notes"
+                  class="mt-1 text-xs text-gray-500 dark:text-dark-400 italic max-w-[200px] truncate"
+                  :title="item.notes"
+                >
+                  {{ item.notes }}
+                </p>
               </div>
             </div>
           </div>