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/repository/ops_repo_preagg.go

@@ -71,7 +71,9 @@ usage_agg AS (
 error_base AS (
   SELECT
     date_trunc('hour', created_at AT TIME ZONE 'UTC') AT TIME ZONE 'UTC' AS bucket_start,
-    platform AS platform,
+    -- platform is NULL for some early-phase errors (e.g. before routing); map to a sentinel
+    -- value so platform-level GROUPING SETS don't collide with the overall (platform=NULL) row.
+    COALESCE(platform, 'unknown') AS platform,
     group_id AS group_id,
     is_business_limited AS is_business_limited,
     error_owner AS error_owner,

backend/internal/service/gateway_multiplatform_test.go

@@ -1211,6 +1211,72 @@ func TestGatewayService_SelectAccountWithLoadAwareness(t *testing.T) {
 		require.Nil(t, result)
 		require.Contains(t, err.Error(), "no available accounts")
 	})
+
+	t.Run("过滤不可调度账号-限流账号被跳过", func(t *testing.T) {
+		now := time.Now()
+		resetAt := now.Add(10 * time.Minute)
+
+		repo := &mockAccountRepoForPlatform{
+			accounts: []Account{
+				{ID: 1, Platform: PlatformAnthropic, Priority: 1, Status: StatusActive, Schedulable: true, Concurrency: 5, RateLimitResetAt: &resetAt},
+				{ID: 2, Platform: PlatformAnthropic, Priority: 2, Status: StatusActive, Schedulable: true, Concurrency: 5},
+			},
+			accountsByID: map[int64]*Account{},
+		}
+		for i := range repo.accounts {
+			repo.accountsByID[repo.accounts[i].ID] = &repo.accounts[i]
+		}
+
+		cache := &mockGatewayCacheForPlatform{}
+		cfg := testConfig()
+		cfg.Gateway.Scheduling.LoadBatchEnabled = false
+
+		svc := &GatewayService{
+			accountRepo:        repo,
+			cache:              cache,
+			cfg:                cfg,
+			concurrencyService: nil,
+		}
+
+		result, err := svc.SelectAccountWithLoadAwareness(ctx, nil, "", "claude-3-5-sonnet-20241022", nil)
+		require.NoError(t, err)
+		require.NotNil(t, result)
+		require.NotNil(t, result.Account)
+		require.Equal(t, int64(2), result.Account.ID, "应跳过限流账号，选择可用账号")
+	})
+
+	t.Run("过滤不可调度账号-过载账号被跳过", func(t *testing.T) {
+		now := time.Now()
+		overloadUntil := now.Add(10 * time.Minute)
+
+		repo := &mockAccountRepoForPlatform{
+			accounts: []Account{
+				{ID: 1, Platform: PlatformAnthropic, Priority: 1, Status: StatusActive, Schedulable: true, Concurrency: 5, OverloadUntil: &overloadUntil},
+				{ID: 2, Platform: PlatformAnthropic, Priority: 2, Status: StatusActive, Schedulable: true, Concurrency: 5},
+			},
+			accountsByID: map[int64]*Account{},
+		}
+		for i := range repo.accounts {
+			repo.accountsByID[repo.accounts[i].ID] = &repo.accounts[i]
+		}
+
+		cache := &mockGatewayCacheForPlatform{}
+		cfg := testConfig()
+		cfg.Gateway.Scheduling.LoadBatchEnabled = false
+
+		svc := &GatewayService{
+			accountRepo:        repo,
+			cache:              cache,
+			cfg:                cfg,
+			concurrencyService: nil,
+		}
+
+		result, err := svc.SelectAccountWithLoadAwareness(ctx, nil, "", "claude-3-5-sonnet-20241022", nil)
+		require.NoError(t, err)
+		require.NotNil(t, result)
+		require.NotNil(t, result.Account)
+		require.Equal(t, int64(2), result.Account.ID, "应跳过过载账号，选择可用账号")
+	})
 }
 
 func TestGatewayService_GroupResolution_ReusesContextGroup(t *testing.T) {

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

backend/internal/service/openai_gateway_service.go

@@ -186,6 +186,11 @@ func (s *OpenAIGatewayService) SelectAccountForModelWithExclusions(ctx context.C
 		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
+		}
 		// Check model support
 		if requestedModel != "" && !acc.IsModelSupported(requestedModel) {
 			continue
@@ -332,6 +337,12 @@ func (s *OpenAIGatewayService) SelectAccountWithLoadAwareness(ctx context.Contex
 		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 requestedModel != "" && !acc.IsModelSupported(requestedModel) {
 			continue
 		}

backend/internal/service/openai_gateway_service_test.go

@@ -3,6 +3,7 @@ package service
 import (
 	"bufio"
 	"bytes"
+	"context"
 	"errors"
 	"io"
 	"net/http"
@@ -15,6 +16,129 @@ import (
 	"github.com/gin-gonic/gin"
 )
 
+type stubOpenAIAccountRepo struct {
+	AccountRepository
+	accounts []Account
+}
+
+func (r stubOpenAIAccountRepo) ListSchedulableByGroupIDAndPlatform(ctx context.Context, groupID int64, platform string) ([]Account, error) {
+	return append([]Account(nil), r.accounts...), nil
+}
+
+func (r stubOpenAIAccountRepo) ListSchedulableByPlatform(ctx context.Context, platform string) ([]Account, error) {
+	return append([]Account(nil), r.accounts...), nil
+}
+
+type stubConcurrencyCache struct {
+	ConcurrencyCache
+}
+
+func (c stubConcurrencyCache) AcquireAccountSlot(ctx context.Context, accountID int64, maxConcurrency int, requestID string) (bool, error) {
+	return true, nil
+}
+
+func (c stubConcurrencyCache) ReleaseAccountSlot(ctx context.Context, accountID int64, requestID string) error {
+	return nil
+}
+
+func (c stubConcurrencyCache) GetAccountsLoadBatch(ctx context.Context, accounts []AccountWithConcurrency) (map[int64]*AccountLoadInfo, error) {
+	out := make(map[int64]*AccountLoadInfo, len(accounts))
+	for _, acc := range accounts {
+		out[acc.ID] = &AccountLoadInfo{AccountID: acc.ID, LoadRate: 0}
+	}
+	return out, nil
+}
+
+func TestOpenAISelectAccountWithLoadAwareness_FiltersUnschedulable(t *testing.T) {
+	now := time.Now()
+	resetAt := now.Add(10 * time.Minute)
+	groupID := int64(1)
+
+	rateLimited := Account{
+		ID:               1,
+		Platform:         PlatformOpenAI,
+		Type:             AccountTypeAPIKey,
+		Status:           StatusActive,
+		Schedulable:      true,
+		Concurrency:      1,
+		Priority:         0,
+		RateLimitResetAt: &resetAt,
+	}
+	available := Account{
+		ID:          2,
+		Platform:    PlatformOpenAI,
+		Type:        AccountTypeAPIKey,
+		Status:      StatusActive,
+		Schedulable: true,
+		Concurrency: 1,
+		Priority:    1,
+	}
+
+	svc := &OpenAIGatewayService{
+		accountRepo:        stubOpenAIAccountRepo{accounts: []Account{rateLimited, available}},
+		concurrencyService: NewConcurrencyService(stubConcurrencyCache{}),
+	}
+
+	selection, err := svc.SelectAccountWithLoadAwareness(context.Background(), &groupID, "", "gpt-5.2", nil)
+	if err != nil {
+		t.Fatalf("SelectAccountWithLoadAwareness error: %v", err)
+	}
+	if selection == nil || selection.Account == nil {
+		t.Fatalf("expected selection with account")
+	}
+	if selection.Account.ID != available.ID {
+		t.Fatalf("expected account %d, got %d", available.ID, selection.Account.ID)
+	}
+	if selection.ReleaseFunc != nil {
+		selection.ReleaseFunc()
+	}
+}
+
+func TestOpenAISelectAccountWithLoadAwareness_FiltersUnschedulableWhenNoConcurrencyService(t *testing.T) {
+	now := time.Now()
+	resetAt := now.Add(10 * time.Minute)
+	groupID := int64(1)
+
+	rateLimited := Account{
+		ID:               1,
+		Platform:         PlatformOpenAI,
+		Type:             AccountTypeAPIKey,
+		Status:           StatusActive,
+		Schedulable:      true,
+		Concurrency:      1,
+		Priority:         0,
+		RateLimitResetAt: &resetAt,
+	}
+	available := Account{
+		ID:          2,
+		Platform:    PlatformOpenAI,
+		Type:        AccountTypeAPIKey,
+		Status:      StatusActive,
+		Schedulable: true,
+		Concurrency: 1,
+		Priority:    1,
+	}
+
+	svc := &OpenAIGatewayService{
+		accountRepo: stubOpenAIAccountRepo{accounts: []Account{rateLimited, available}},
+		// concurrencyService is nil, forcing the non-load-batch selection path.
+	}
+
+	selection, err := svc.SelectAccountWithLoadAwareness(context.Background(), &groupID, "", "gpt-5.2", nil)
+	if err != nil {
+		t.Fatalf("SelectAccountWithLoadAwareness error: %v", err)
+	}
+	if selection == nil || selection.Account == nil {
+		t.Fatalf("expected selection with account")
+	}
+	if selection.Account.ID != available.ID {
+		t.Fatalf("expected account %d, got %d", available.ID, selection.Account.ID)
+	}
+	if selection.ReleaseFunc != nil {
+		selection.ReleaseFunc()
+	}
+}
+
 func TestOpenAIStreamingTimeout(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	cfg := &config.Config{