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.go

@@ -45,6 +45,12 @@ type Group struct {
 	MonthlyLimitUsd *float64 `json:"monthly_limit_usd,omitempty"`
 	// DefaultValidityDays holds the value of the "default_validity_days" field.
 	DefaultValidityDays int `json:"default_validity_days,omitempty"`
+	// ImagePrice1k holds the value of the "image_price_1k" field.
+	ImagePrice1k *float64 `json:"image_price_1k,omitempty"`
+	// ImagePrice2k holds the value of the "image_price_2k" field.
+	ImagePrice2k *float64 `json:"image_price_2k,omitempty"`
+	// ImagePrice4k holds the value of the "image_price_4k" field.
+	ImagePrice4k *float64 `json:"image_price_4k,omitempty"`
 	// Edges holds the relations/edges for other nodes in the graph.
 	// The values are being populated by the GroupQuery when eager-loading is set.
 	Edges        GroupEdges `json:"edges"`
@@ -153,7 +159,7 @@ func (*Group) scanValues(columns []string) ([]any, error) {
 		switch columns[i] {
 		case group.FieldIsExclusive:
 			values[i] = new(sql.NullBool)
-		case group.FieldRateMultiplier, group.FieldDailyLimitUsd, group.FieldWeeklyLimitUsd, group.FieldMonthlyLimitUsd:
+		case group.FieldRateMultiplier, group.FieldDailyLimitUsd, group.FieldWeeklyLimitUsd, group.FieldMonthlyLimitUsd, group.FieldImagePrice1k, group.FieldImagePrice2k, group.FieldImagePrice4k:
 			values[i] = new(sql.NullFloat64)
 		case group.FieldID, group.FieldDefaultValidityDays:
 			values[i] = new(sql.NullInt64)
@@ -271,6 +277,27 @@ func (_m *Group) assignValues(columns []string, values []any) error {
 			} else if value.Valid {
 				_m.DefaultValidityDays = int(value.Int64)
 			}
+		case group.FieldImagePrice1k:
+			if value, ok := values[i].(*sql.NullFloat64); !ok {
+				return fmt.Errorf("unexpected type %T for field image_price_1k", values[i])
+			} else if value.Valid {
+				_m.ImagePrice1k = new(float64)
+				*_m.ImagePrice1k = value.Float64
+			}
+		case group.FieldImagePrice2k:
+			if value, ok := values[i].(*sql.NullFloat64); !ok {
+				return fmt.Errorf("unexpected type %T for field image_price_2k", values[i])
+			} else if value.Valid {
+				_m.ImagePrice2k = new(float64)
+				*_m.ImagePrice2k = value.Float64
+			}
+		case group.FieldImagePrice4k:
+			if value, ok := values[i].(*sql.NullFloat64); !ok {
+				return fmt.Errorf("unexpected type %T for field image_price_4k", values[i])
+			} else if value.Valid {
+				_m.ImagePrice4k = new(float64)
+				*_m.ImagePrice4k = value.Float64
+			}
 		default:
 			_m.selectValues.Set(columns[i], values[i])
 		}
@@ -398,6 +425,21 @@ func (_m *Group) String() string {
 	builder.WriteString(", ")
 	builder.WriteString("default_validity_days=")
 	builder.WriteString(fmt.Sprintf("%v", _m.DefaultValidityDays))
+	builder.WriteString(", ")
+	if v := _m.ImagePrice1k; v != nil {
+		builder.WriteString("image_price_1k=")
+		builder.WriteString(fmt.Sprintf("%v", *v))
+	}
+	builder.WriteString(", ")
+	if v := _m.ImagePrice2k; v != nil {
+		builder.WriteString("image_price_2k=")
+		builder.WriteString(fmt.Sprintf("%v", *v))
+	}
+	builder.WriteString(", ")
+	if v := _m.ImagePrice4k; v != nil {
+		builder.WriteString("image_price_4k=")
+		builder.WriteString(fmt.Sprintf("%v", *v))
+	}
 	builder.WriteByte(')')
 	return builder.String()
 }