feat(backend): 购买单取消透传 pay v2 cancel

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/handler/pay.go

@@ -91,6 +91,27 @@ func (h *PayHandler) PurchaseStatus(c *gin.Context) {
 	util.RespondSuccess(c, st)
 }
 
+// Cancel POST /api/v1/license/purchase/:out_trade_no/cancel — 取消本店一笔购买单
+// （防 pending 单堆积挤占 reconcile 每轮 50 条限额）。仅管理员可用（handler 内判权，同 Purchase）。
+func (h *PayHandler) Cancel(c *gin.Context) {
+	role := middleware.GetRole(c)
+	if role != "admin" && role != "superadmin" {
+		c.JSON(http.StatusForbidden, gin.H{"error": "仅管理员可取消购买单"})
+		return
+	}
+	canceled, err := h.svc.CancelPurchase(middleware.GetShopID(c), c.Param("out_trade_no"))
+	if err != nil {
+		if errors.Is(err, service.ErrPurchaseNotFound) {
+			c.JSON(http.StatusNotFound, gin.H{"error": err.Error()})
+			return
+		}
+		log.Printf("[pay] cancel failed shop=%d out_trade_no=%s: %v", middleware.GetShopID(c), c.Param("out_trade_no"), err)
+		c.JSON(http.StatusBadGateway, gin.H{"error": "取消失败，请稍后重试"})
+		return
+	}
+	util.RespondSuccess(c, gin.H{"canceled": canceled})
+}
+
 // PromoStatus GET /api/v1/license/promo-status — 本店首月特惠是否已享用（前端据此置灰特惠档）。
 func (h *PayHandler) PromoStatus(c *gin.Context) {
 	used, err := h.svc.PromoUsed(middleware.GetShopID(c))