fix: use range assertion for time-sensitive ExpiresInDays test

The test could flake depending on exact execution time near midnight
boundaries. Use a range check (29 or 30) instead of exact equality.

backend/internal/service/subscription_calculate_progress_test.go

@@ -34,7 +34,7 @@ func TestCalculateProgress_BasicFields(t *testing.T) {
 	assert.Equal(t, int64(100), progress.ID)
 	assert.Equal(t, "Premium", progress.GroupName)
 	assert.Equal(t, sub.ExpiresAt, progress.ExpiresAt)
-	assert.Equal(t, 29, progress.ExpiresInDays) // 约 30 天
+	assert.True(t, progress.ExpiresInDays == 29 || progress.ExpiresInDays == 30, "ExpiresInDays should be 29 or 30, got %d", progress.ExpiresInDays)
 	assert.Nil(t, progress.Daily, "无日限额时 Daily 应为 nil")
 	assert.Nil(t, progress.Weekly, "无周限额时 Weekly 应为 nil")
 	assert.Nil(t, progress.Monthly, "无月限额时 Monthly 应为 nil")