fix(网关): 修复账号选择中的调度器快照延迟问题

## 问题描述
调度器快照更新存在0.5-1秒的延迟（Outbox轮询间隔），导致在账号被限流或过载后的短时间窗口内，
可能仍会被选中，造成请求失败。

## 根本原因
账号选择逻辑依赖调度器快照（listSchedulableAccounts），但快照更新有延迟：
- Outbox轮询: 每1秒检查一次变更事件
- 全量重建: 每300秒重建一次
- 时间窗口: 账号状态变更后0.5-1秒内，快照可能未更新

## 解决方案
在账号选择循环中添加IsSchedulable()实时检查，作为第二道防线：
1. 第一道防线: 调度器快照过滤（可能有延迟）
2. 第二道防线: IsSchedulable()实时检查（本次修复）

IsSchedulable()会检查：
- RateLimitResetAt: 限流重置时间
- OverloadUntil: 过载持续时间
- TempUnschedulableUntil: 临时不可调度时间
- Status: 账号状态
- Schedulable: 可调度标志

## 修改范围
### OpenAI Gateway Service
- SelectAccountForModelWithExclusions: 添加IsSchedulable()检查
- SelectAccountWithLoadAwareness: 添加IsSchedulable()检查

### Gateway Service (Claude/Gemini/Antigravity)
- 负载感知选择候选账号筛选: 添加IsSchedulable()检查
- selectAccountForModelWithPlatform: 添加IsSchedulable()检查
- selectAccountWithMixedScheduling: 添加IsSchedulable()检查

### 测试用例
- OpenAI: 添加2个测试用例验证限流账号过滤
- Gateway: 添加2个测试用例验证限流和过载账号过滤

### 其他修复
- ops_repo_preagg.go: 修复platform为NULL时的聚合问题

## 测试结果
所有单元测试通过 ✅

backend/internal/service/gateway_service.go

@@ -511,6 +511,12 @@ func (s *GatewayService) SelectAccountWithLoadAwareness(ctx context.Context, gro
 		if isExcluded(acc.ID) {
 			continue
 		}
+		// Scheduler snapshots can be temporarily stale (bucket rebuild is throttled);
+		// re-check schedulability here so recently rate-limited/overloaded accounts
+		// are not selected again before the bucket is rebuilt.
+		if !acc.IsSchedulable() {
+			continue
+		}
 		if !s.isAccountAllowedForPlatform(acc, platform, useMixed) {
 			continue
 		}
@@ -893,6 +899,11 @@ func (s *GatewayService) selectAccountForModelWithPlatform(ctx context.Context,
 		if _, excluded := excludedIDs[acc.ID]; excluded {
 			continue
 		}
+		// Scheduler snapshots can be temporarily stale; re-check schedulability here to
+		// avoid selecting accounts that were recently rate-limited/overloaded.
+		if !acc.IsSchedulable() {
+			continue
+		}
 		if !acc.IsSchedulableForModel(requestedModel) {
 			continue
 		}
@@ -977,6 +988,11 @@ func (s *GatewayService) selectAccountWithMixedScheduling(ctx context.Context, g
 		if _, excluded := excludedIDs[acc.ID]; excluded {
 			continue
 		}
+		// Scheduler snapshots can be temporarily stale; re-check schedulability here to
+		// avoid selecting accounts that were recently rate-limited/overloaded.
+		if !acc.IsSchedulable() {
+			continue
+		}
 		// 过滤：原生平台直接通过，antigravity 需要启用混合调度
 		if acc.Platform == PlatformAntigravity && !acc.IsMixedSchedulingEnabled() {
 			continue