feat(v2): Order/Attempt/Account/Refund 模型 + 状态枚举 + :memory: 测试约定

v2 OrderStatus 三个值(Pending/Paid/Refunded)与 v1 model.OrderStatus 撞名,
统一加 V2 后缀(OrderPendingV2 等)规避包内重名;AttemptStatus/RefundStatus
无此冲突,原样保留。

internal/model/testdb.go

@@ -0,0 +1,26 @@
+package model
+
+import (
+	"testing"
+
+	"github.com/glebarez/sqlite"
+	"gorm.io/gorm"
+	"gorm.io/gorm/logger"
+)
+
+// OpenTestDB opens an in-memory SQLite DB with the v2 schema migrated.
+// Shared by all package tests (pay has no prior DB test harness — this is it).
+func OpenTestDB(t *testing.T) *gorm.DB {
+	t.Helper()
+	db, err := gorm.Open(sqlite.Open("file::memory:?cache=shared"),
+		&gorm.Config{Logger: logger.Default.LogMode(logger.Silent), TranslateError: true})
+	if err != nil {
+		t.Fatalf("open test db: %v", err)
+	}
+	if err := db.AutoMigrate(&OrderV2{}, &Attempt{}, &Account{}, &Refund{}); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+	sqlDB, _ := db.DB()
+	t.Cleanup(func() { _ = sqlDB.Close() })
+	return db
+}