修复：隔离分组用量仓储测试时区

分组用量仓储测试原先固定恢复上海时区，覆盖集成测试包初始化的 UTC，并导致后续今日统计多算。统一保存并恢复测试进入前的时区，避免 unit 和 integration 用例相互污染。

backend/internal/repository/custom_group_usage_timezone_test.go

@@ -0,0 +1,16 @@
+package repository
+
+import (
+	"testing"
+
+	appTimezone "github.com/Wei-Shaw/sub2api/internal/pkg/timezone"
+	"github.com/stretchr/testify/require"
+)
+
+func useGroupUsageRepositoryTestTimezone(t *testing.T, name string) {
+	t.Helper()
+
+	previousName := appTimezone.Name()
+	require.NoError(t, appTimezone.Init(name))
+	t.Cleanup(func() { require.NoError(t, appTimezone.Init(previousName)) })
+}

backend/internal/repository/dashboard_aggregation_group_usage_test.go

@@ -10,7 +10,6 @@ import (
 	"time"
 
 	"github.com/DATA-DOG/go-sqlmock"
-	appTimezone "github.com/Wei-Shaw/sub2api/internal/pkg/timezone"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 	"github.com/stretchr/testify/require"
 )
@@ -32,8 +31,7 @@ func TestDashboardAggregationRepositorySyncGroupUsageRollupsNoopsAtCurrentDate(t
 }
 
 func TestDashboardAggregationRepositorySyncGroupUsageRollupsRebuildsWhenTimezoneChanges(t *testing.T) {
-	require.NoError(t, appTimezone.Init("America/New_York"))
-	t.Cleanup(func() { require.NoError(t, appTimezone.Init("Asia/Shanghai")) })
+	useGroupUsageRepositoryTestTimezone(t, "America/New_York")
 
 	db, mock := newSQLMock(t)
 	repo := newDashboardAggregationRepositoryWithSQL(db)
@@ -317,6 +315,5 @@ func TestDashboardAggregationRepositoryCleanupUsageLogsPartitionFailureRollsBack
 
 func setGroupUsageRollupTestTimezone(t *testing.T) {
 	t.Helper()
-	require.NoError(t, appTimezone.Init("Asia/Shanghai"))
-	t.Cleanup(func() { require.NoError(t, appTimezone.Init("Asia/Shanghai")) })
+	useGroupUsageRepositoryTestTimezone(t, "Asia/Shanghai")
 }

backend/internal/repository/group_usage_rollup_trigger_integration_test.go

@@ -9,7 +9,6 @@ import (
 	"testing"
 	"time"
 
-	appTimezone "github.com/Wei-Shaw/sub2api/internal/pkg/timezone"
 	"github.com/Wei-Shaw/sub2api/migrations"
 	"github.com/lib/pq"
 	"github.com/stretchr/testify/require"
@@ -259,8 +258,7 @@ func TestGroupUsageRollupTriggerUsesSessionTimezoneAcrossDST(t *testing.T) {
 
 func TestGroupUsageSummaryIncludesYesterdayAcrossWatermark(t *testing.T) {
 	ctx := context.Background()
-	require.NoError(t, appTimezone.Init("Asia/Shanghai"))
-	t.Cleanup(func() { require.NoError(t, appTimezone.Init("Asia/Shanghai")) })
+	useGroupUsageRepositoryTestTimezone(t, "Asia/Shanghai")
 	todayStart := time.Date(2026, 8, 13, 16, 0, 0, 0, time.UTC)
 
 	tests := []struct {
@@ -317,8 +315,7 @@ func TestGroupUsageSummaryIncludesYesterdayAcrossWatermark(t *testing.T) {
 
 func TestGroupUsageRollupSyncRebuildsAfterTimezoneChange(t *testing.T) {
 	ctx := context.Background()
-	require.NoError(t, appTimezone.Init("America/New_York"))
-	t.Cleanup(func() { require.NoError(t, appTimezone.Init("Asia/Shanghai")) })
+	useGroupUsageRepositoryTestTimezone(t, "America/New_York")
 	todayStart := time.Date(2026, 3, 9, 4, 0, 0, 0, time.UTC)
 	schema := createGroupUsageRollupTriggerTestSchema(t, ctx, false)
 	tx := beginGroupUsageRollupTriggerTestTx(t, ctx, schema)
@@ -373,8 +370,7 @@ func TestGroupUsageRollupSyncRebuildsAfterTimezoneChange(t *testing.T) {
 
 func TestGroupUsageSummaryUsesConfiguredDSTBoundaries(t *testing.T) {
 	ctx := context.Background()
-	require.NoError(t, appTimezone.Init("America/New_York"))
-	t.Cleanup(func() { require.NoError(t, appTimezone.Init("Asia/Shanghai")) })
+	useGroupUsageRepositoryTestTimezone(t, "America/New_York")
 	todayStart := time.Date(2026, 3, 9, 4, 0, 0, 0, time.UTC)
 	schema := createGroupUsageRollupTriggerTestSchema(t, ctx, false)
 	tx := beginGroupUsageRollupTriggerTestTx(t, ctx, schema)

backend/internal/repository/usage_cleanup_repo_test.go

@@ -9,7 +9,6 @@ import (
 
 	"github.com/DATA-DOG/go-sqlmock"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/pagination"
-	appTimezone "github.com/Wei-Shaw/sub2api/internal/pkg/timezone"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 	"github.com/stretchr/testify/require"
 )
@@ -24,8 +23,7 @@ func newSQLMock(t *testing.T) (*sql.DB, sqlmock.Sqlmock) {
 
 func setUsageCleanupRollupTestTimezone(t *testing.T) {
 	t.Helper()
-	require.NoError(t, appTimezone.Init("Asia/Shanghai"))
-	t.Cleanup(func() { require.NoError(t, appTimezone.Init("Asia/Shanghai")) })
+	useGroupUsageRepositoryTestTimezone(t, "Asia/Shanghai")
 }
 
 func TestNewUsageCleanupRepository(t *testing.T) {

backend/internal/repository/usage_log_repo_group_summary_test.go

@@ -8,15 +8,13 @@ import (
 	"time"
 
 	"github.com/DATA-DOG/go-sqlmock"
-	appTimezone "github.com/Wei-Shaw/sub2api/internal/pkg/timezone"
 	"github.com/stretchr/testify/require"
 )
 
 func TestUsageLogRepositoryGetAllGroupUsageSummaryUsesRollupTail(t *testing.T) {
 	db, mock := newSQLMock(t)
 	repo := newUsageLogRepositoryWithSQL(nil, db)
-	require.NoError(t, appTimezone.Init("America/New_York"))
-	t.Cleanup(func() { require.NoError(t, appTimezone.Init("Asia/Shanghai")) })
+	useGroupUsageRepositoryTestTimezone(t, "America/New_York")
 	todayStart := time.Date(2026, 3, 9, 4, 0, 0, 0, time.UTC)
 	yesterdayStart := time.Date(2026, 3, 8, 5, 0, 0, 0, time.UTC)