修复：软删除账号可复用用户名

backend/internal/service/merchant_test.go

@@ -373,3 +373,26 @@ func TestDeleteMerchantAccountRemovesAllMemberships(t *testing.T) {
 		t.Fatal("platform administrator should not be deletable from merchant account endpoint")
 	}
 }
+
+func TestUsernameCanBeReusedAfterMerchantAccountSoftDelete(t *testing.T) {
+	db := newServiceTestDB(t)
+	svc := NewUserService(db, nil)
+	admin := model.User{Username: "reuse-account-admin", PasswordHash: "hash", Role: model.RoleAdmin, Status: 1}
+	if err := db.Create(&admin).Error; err != nil {
+		t.Fatalf("create admin: %v", err)
+	}
+	account, err := svc.Create("reusable-username", "password123", "旧账号", model.RoleMerchant, 0)
+	if err != nil {
+		t.Fatalf("create merchant account: %v", err)
+	}
+	if err := svc.DeleteMerchantAccount(account.ID, admin.ID); err != nil {
+		t.Fatalf("soft delete merchant account: %v", err)
+	}
+	recreated, err := svc.Create("reusable-username", "password123", "新账号", model.RoleMerchant, 0)
+	if err != nil {
+		t.Fatalf("recreate account with soft-deleted username: %v", err)
+	}
+	if recreated.ID == account.ID {
+		t.Fatalf("expected a new account record, got id=%d", recreated.ID)
+	}
+}