feat: add TenantLedger model and query generation

- Introduced TenantLedger model with fields for managing tenant transactions, including ID, TenantID, UserID, OrderID, transaction Type, Amount, and balance details.
- Implemented CRUD operations for TenantLedger with methods for Create, Update, Delete, and Reload.
- Generated query methods for TenantLedger to facilitate database interactions, including filtering, pagination, and aggregation functions.
- Established relationships with Order model for foreign key references.

backend/database/models/content_prices.query.gen.go

@@ -47,18 +47,18 @@ type contentPriceQuery struct {
 	contentPriceQueryDo contentPriceQueryDo
 
 	ALL             field.Asterisk
-	ID              field.Int64
-	TenantID        field.Int64
-	UserID          field.Int64
-	ContentID       field.Int64
-	Currency        field.Field
-	PriceAmount     field.Int64
-	DiscountType    field.Field
-	DiscountValue   field.Int64
-	DiscountStartAt field.Time
-	DiscountEndAt   field.Time
-	CreatedAt       field.Time
-	UpdatedAt       field.Time
+	ID              field.Int64 // 主键ID：自增
+	TenantID        field.Int64 // 租户ID：多租户隔离；与内容归属一致
+	UserID          field.Int64 // 用户ID：设置/更新价格的操作人（通常为 tenant_admin）；用于审计
+	ContentID       field.Int64 // 内容ID：唯一约束 (tenant_id, content_id)；一个内容在一个租户内仅一份定价
+	Currency        field.Field // 币种：当前固定 CNY；金额单位为分
+	PriceAmount     field.Int64 // 基础价格：分；0 表示免费（可直接访问正片资源）
+	DiscountType    field.Field // 折扣类型：none/percent/amount；仅影响下单时成交价，需写入订单快照
+	DiscountValue   field.Int64 // 折扣值：percent=0-100（按业务校验）；amount=分；none 时忽略
+	DiscountStartAt field.Time  // 折扣开始时间：可为空；为空表示立即生效（由业务逻辑解释）
+	DiscountEndAt   field.Time  // 折扣结束时间：可为空；为空表示长期有效（由业务逻辑解释）
+	CreatedAt       field.Time  // 创建时间：默认 now()；用于审计
+	UpdatedAt       field.Time  // 更新时间：默认 now()；更新价格/折扣时写入
 
 	fieldMap map[string]field.Expr
 }