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/runtime/runtime.go

@@ -521,8 +521,16 @@ func init() {
 	usagelogDescStream := usagelogFields[21].Descriptor()
 	// usagelog.DefaultStream holds the default value on creation for the stream field.
 	usagelog.DefaultStream = usagelogDescStream.Default.(bool)
+	// usagelogDescImageCount is the schema descriptor for image_count field.
+	usagelogDescImageCount := usagelogFields[24].Descriptor()
+	// usagelog.DefaultImageCount holds the default value on creation for the image_count field.
+	usagelog.DefaultImageCount = usagelogDescImageCount.Default.(int)
+	// usagelogDescImageSize is the schema descriptor for image_size field.
+	usagelogDescImageSize := usagelogFields[25].Descriptor()
+	// usagelog.ImageSizeValidator is a validator for the "image_size" field. It is called by the builders before save.
+	usagelog.ImageSizeValidator = usagelogDescImageSize.Validators[0].(func(string) error)
 	// usagelogDescCreatedAt is the schema descriptor for created_at field.
-	usagelogDescCreatedAt := usagelogFields[24].Descriptor()
+	usagelogDescCreatedAt := usagelogFields[26].Descriptor()
 	// usagelog.DefaultCreatedAt holds the default value on creation for the created_at field.
 	usagelog.DefaultCreatedAt = usagelogDescCreatedAt.Default.(func() time.Time)
 	userMixin := schema.User{}.Mixin()