积分充值全流程落地：申请审核入账、凭证上传压缩转webp、管理员调账收权

- 新增积分充值申请与审核：商户提交充值申请（凭证图片必填，1元=100积分），
  管理员审核通过后事务内自动入账，幂等键防重复
- 新增通用图片上传组件 ImageUploader：canvas 压缩转 webp、预览、删除，
  后端按魔数校验图片格式并持久化到 data/uploads
- 管理员钱包调整改为按商户维度：移除商户侧调账接口，新增管理员
  平台商户积分调整与钱包流水查看
- 低余额 Webhook 告警配置持久化，充值入账/调账后自动检查推送
- 平台商户操作列移除进入/成员，积分充值页接入真实接口并修复金额单位换算

backend/internal/handler/merchant.go

@@ -17,14 +17,16 @@ type MerchantHandler struct {
 	fulfillmentSvc *service.FulfillmentService
 	callbackSvc    *service.CallbackService
 	deliverySvc    *service.DeliveryService
+	rechargeSvc    *service.RechargeService
 }
 
-func NewMerchantHandler(merchantSvc *service.MerchantService, fulfillmentSvc *service.FulfillmentService, callbackSvc *service.CallbackService, deliverySvc *service.DeliveryService) *MerchantHandler {
+func NewMerchantHandler(merchantSvc *service.MerchantService, fulfillmentSvc *service.FulfillmentService, callbackSvc *service.CallbackService, deliverySvc *service.DeliveryService, rechargeSvc *service.RechargeService) *MerchantHandler {
 	return &MerchantHandler{
 		merchantSvc:    merchantSvc,
 		fulfillmentSvc: fulfillmentSvc,
 		callbackSvc:    callbackSvc,
 		deliverySvc:    deliverySvc,
+		rechargeSvc:    rechargeSvc,
 	}
 }
 
@@ -274,6 +276,7 @@ func (h *MerchantHandler) AdminAdjustWallet(c *gin.Context) {
 		response.BadRequest(c, err.Error())
 		return
 	}
+	h.rechargeSvc.CheckLowBalanceAndNotify(uint(id))
 	response.OK(c, wallet)
 }
 
@@ -359,10 +362,10 @@ func (h *MerchantHandler) ListCallbacks(c *gin.Context) {
 }
 
 type callbackReq struct {
-	Name        string `json:"name"`
-	URL         string `json:"url" binding:"required"`
-	Events      string `json:"events" binding:"required"`
-	Status      string `json:"status"`
+	Name         string `json:"name"`
+	URL          string `json:"url" binding:"required"`
+	Events       string `json:"events" binding:"required"`
+	Status       string `json:"status"`
 	RotateSecret bool   `json:"rotate_secret"`
 }
 
@@ -373,10 +376,10 @@ func (h *MerchantHandler) CreateCallback(c *gin.Context) {
 		return
 	}
 	credential, err := h.callbackSvc.CreateSubscription(middleware.GetMerchantID(c), service.CreateCallbackInput{
-		Name:        req.Name,
-		URL:         req.URL,
-		Events:      req.Events,
-		Status:      req.Status,
+		Name:         req.Name,
+		URL:          req.URL,
+		Events:       req.Events,
+		Status:       req.Status,
 		RotateSecret: req.RotateSecret,
 	}, middleware.GetUserID(c))
 	if err != nil {