fix: 修复批量更新凭证明细与缓存TTL抖动

- BatchUpdateCredentials 返回 success/failed/results 及 success_ids/failed_ids

- billing jitteredTTL 改为只减不增，确保TTL不超上界

- crypto/rand 失败时随机ID降级避免 panic

- OpenAI SelectAccount 失败日志去重并补充字段

- 修复两处类型断言以通过 errcheck

backend/internal/repository/billing_cache.go

@@ -22,8 +22,12 @@ const (
 
 // jitteredTTL 返回带随机抖动的 TTL，防止缓存雪崩
 func jitteredTTL() time.Duration {
-	jitter := time.Duration(rand.Int63n(int64(2*billingCacheJitter))) - billingCacheJitter
-	return billingCacheTTL + jitter
+	// 只做“减法抖动”，确保实际 TTL 不会超过 billingCacheTTL（避免上界预期被打破）。
+	if billingCacheJitter <= 0 {
+		return billingCacheTTL
+	}
+	jitter := time.Duration(rand.Int63n(int64(billingCacheJitter)))
+	return billingCacheTTL - jitter
 }
 
 // billingBalanceKey generates the Redis key for user balance cache.