fix(billing): reject rate_multiplier <= 0 on save; clamp negatives to 0 in compute

分组倍率和用户专属倍率在保存时没有校验，0 会触发计费层的 `<=0 → 1.0`
防御条款，结果订阅/余额分组按标准价扣费；完全是沉默地绕过了业务规则。

- 保存校验（admin_service）：CreateGroup / UpdateGroup / BatchSetGroupRateMultipliers /
  UpdateUser.SyncUserGroupRates 全部要求 > 0
- 计算层（billing_service）：三处 `<=0 → 1.0` 改为 `<0 → 0`；负数按 0 结算，
  避免配置异常被静默按 1x 收费
- 前端：分组倍率 / 用户专属倍率输入 min 统一到 0.001
- 删除未使用的 IsFreeSubscription 方法

测试：新增 billing_service_rate_multiplier_test.go 端到端验证；更新原有锁定
旧 `<=0 → 1.0` 行为的测试。

backend/internal/service/admin_service_group_test.go

@@ -621,6 +621,7 @@ func TestAdminService_CreateGroup_InvalidRequestFallbackRejectsUnsupportedPlatfo
 	_, err := svc.CreateGroup(context.Background(), &CreateGroupInput{
 		Name:                            "g1",
 		Platform:                        PlatformOpenAI,
+		RateMultiplier:                  1.0,
 		SubscriptionType:                SubscriptionTypeStandard,
 		FallbackGroupIDOnInvalidRequest: &fallbackID,
 	})
@@ -641,6 +642,7 @@ func TestAdminService_CreateGroup_InvalidRequestFallbackRejectsSubscription(t *t
 	_, err := svc.CreateGroup(context.Background(), &CreateGroupInput{
 		Name:                            "g1",
 		Platform:                        PlatformAnthropic,
+		RateMultiplier:                  1.0,
 		SubscriptionType:                SubscriptionTypeSubscription,
 		FallbackGroupIDOnInvalidRequest: &fallbackID,
 	})
@@ -695,6 +697,7 @@ func TestAdminService_CreateGroup_InvalidRequestFallbackRejectsFallbackGroup(t *
 			_, err := svc.CreateGroup(context.Background(), &CreateGroupInput{
 				Name:                            "g1",
 				Platform:                        PlatformAnthropic,
+				RateMultiplier:                  1.0,
 				SubscriptionType:                SubscriptionTypeStandard,
 				FallbackGroupIDOnInvalidRequest: &fallbackID,
 			})
@@ -713,6 +716,7 @@ func TestAdminService_CreateGroup_InvalidRequestFallbackNotFound(t *testing.T) {
 	_, err := svc.CreateGroup(context.Background(), &CreateGroupInput{
 		Name:                            "g1",
 		Platform:                        PlatformAnthropic,
+		RateMultiplier:                  1.0,
 		SubscriptionType:                SubscriptionTypeStandard,
 		FallbackGroupIDOnInvalidRequest: &fallbackID,
 	})
@@ -733,6 +737,7 @@ func TestAdminService_CreateGroup_InvalidRequestFallbackAllowsAntigravity(t *tes
 	group, err := svc.CreateGroup(context.Background(), &CreateGroupInput{
 		Name:                            "g1",
 		Platform:                        PlatformAntigravity,
+		RateMultiplier:                  1.0,
 		SubscriptionType:                SubscriptionTypeStandard,
 		FallbackGroupIDOnInvalidRequest: &fallbackID,
 	})
@@ -750,6 +755,7 @@ func TestAdminService_CreateGroup_InvalidRequestFallbackClearsOnZero(t *testing.
 	group, err := svc.CreateGroup(context.Background(), &CreateGroupInput{
 		Name:                            "g1",
 		Platform:                        PlatformAnthropic,
+		RateMultiplier:                  1.0,
 		SubscriptionType:                SubscriptionTypeStandard,
 		FallbackGroupIDOnInvalidRequest: &zero,
 	})