audit full project flows and harden backend

backend/api/internal/logic/payment/callback_test.go

@@ -0,0 +1,21 @@
+package payment
+
+import (
+	"errors"
+	"testing"
+
+	"gorm.io/gorm"
+)
+
+func TestRequireSingleBusinessUpdate(t *testing.T) {
+	if err := requireSingleBusinessUpdate(&gorm.DB{RowsAffected: 1}); err != nil {
+		t.Fatalf("single state update rejected: %v", err)
+	}
+	if err := requireSingleBusinessUpdate(&gorm.DB{}); err == nil {
+		t.Fatal("zero-row state update must be rejected")
+	}
+	expected := errors.New("database unavailable")
+	if err := requireSingleBusinessUpdate(&gorm.DB{Error: expected}); !errors.Is(err, expected) {
+		t.Fatalf("database error lost: %v", err)
+	}
+}