feat: 图片生成计费功能

- 新增 Group 图片价格配置（image_price_1k/2k/4k）
- BillingService 新增 CalculateImageCost 方法
- AntigravityGatewayService 支持识别图片生成模型并按次计费
- UsageLog 新增 image_count 和 image_size 字段
- 前端分组管理支持配置图片价格（antigravity 和 gemini 平台）
- 图片计费复用通用计费能力（余额检查、扣费、倍率、订阅限额）

backend/ent/group/group.go

@@ -43,6 +43,12 @@ const (
 	FieldMonthlyLimitUsd = "monthly_limit_usd"
 	// FieldDefaultValidityDays holds the string denoting the default_validity_days field in the database.
 	FieldDefaultValidityDays = "default_validity_days"
+	// FieldImagePrice1k holds the string denoting the image_price_1k field in the database.
+	FieldImagePrice1k = "image_price_1k"
+	// FieldImagePrice2k holds the string denoting the image_price_2k field in the database.
+	FieldImagePrice2k = "image_price_2k"
+	// FieldImagePrice4k holds the string denoting the image_price_4k field in the database.
+	FieldImagePrice4k = "image_price_4k"
 	// EdgeAPIKeys holds the string denoting the api_keys edge name in mutations.
 	EdgeAPIKeys = "api_keys"
 	// EdgeRedeemCodes holds the string denoting the redeem_codes edge name in mutations.
@@ -132,6 +138,9 @@ var Columns = []string{
 	FieldWeeklyLimitUsd,
 	FieldMonthlyLimitUsd,
 	FieldDefaultValidityDays,
+	FieldImagePrice1k,
+	FieldImagePrice2k,
+	FieldImagePrice4k,
 }
 
 var (
@@ -267,6 +276,21 @@ func ByDefaultValidityDays(opts ...sql.OrderTermOption) OrderOption {
 	return sql.OrderByField(FieldDefaultValidityDays, opts...).ToFunc()
 }
 
+// ByImagePrice1k orders the results by the image_price_1k field.
+func ByImagePrice1k(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldImagePrice1k, opts...).ToFunc()
+}
+
+// ByImagePrice2k orders the results by the image_price_2k field.
+func ByImagePrice2k(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldImagePrice2k, opts...).ToFunc()
+}
+
+// ByImagePrice4k orders the results by the image_price_4k field.
+func ByImagePrice4k(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldImagePrice4k, opts...).ToFunc()
+}
+
 // ByAPIKeysCount orders the results by api_keys count.
 func ByAPIKeysCount(opts ...sql.OrderTermOption) OrderOption {
 	return func(s *sql.Selector) {