feat: Refactor order snapshot handling and introduce structured snapshot types

- Added new structured snapshot types for orders and order items to improve data integrity and clarity.
- Updated the Order and OrderItem models to use the new JSONType for snapshots.
- Refactored tests to accommodate the new snapshot structure, ensuring compatibility with legacy data.
- Enhanced the OrdersSnapshot struct to support multiple snapshot types and maintain backward compatibility.
- Introduced new fields for order items and orders to capture detailed snapshot information for auditing and historical display.

backend/database/fields/order_items.go

@@ -0,0 +1,15 @@
+package fields
+
+// OrderItemsSnapshot 定义 order_items.snapshot 的固定结构。
+//
+// 该快照用于“历史展示与审计”：即使内容标题/作者等信息后续变更，也不影响已下单记录的展示一致性。
+type OrderItemsSnapshot struct {
+	// ContentID 内容 ID。
+	ContentID int64 `json:"content_id"`
+	// ContentTitle 内容标题快照。
+	ContentTitle string `json:"content_title"`
+	// ContentUserID 内容作者用户 ID。
+	ContentUserID int64 `json:"content_user_id"`
+	// AmountPaid 该行实付金额（分）。
+	AmountPaid int64 `json:"amount_paid"`
+}