fix(gateway): 修复账号跨分组调度问题

问题：账号可能被调度到未分配的分组（如 simon 账号被调度到 claude_default）

根因：
- 强制平台模式下分组查询失败时回退到全平台查询
- listSchedulableAccounts 中分组为空时回退到无分组查询
- 粘性会话只检查平台匹配，未校验账号分组归属

修复：
- 移除强制平台模式的回退逻辑，分组内无账号时返回错误
- 移除 listSchedulableAccounts 的回退逻辑
- 新增 isAccountInGroup 方法用于分组校验
- 在三处粘性会话检查中增加分组归属验证

backend/internal/service/gateway_service.go

@@ -366,17 +366,8 @@ func (s *GatewayService) SelectAccountForModelWithExclusions(ctx context.Context
 		return s.selectAccountWithMixedScheduling(ctx, groupID, sessionHash, requestedModel, excludedIDs, platform)
 	}
 
-	// 强制平台模式：优先按分组查找，找不到再查全部该平台账户
-	if hasForcePlatform && groupID != nil {
-		account, err := s.selectAccountForModelWithPlatform(ctx, groupID, sessionHash, requestedModel, excludedIDs, platform)
-		if err == nil {
-			return account, nil
-		}
-		// 分组中找不到，回退查询全部该平台账户
-		groupID = nil
-	}
-
 	// antigravity 分组、强制平台模式或无分组使用单平台选择
+	// 注意：强制平台模式也必须遵守分组限制，不再回退到全平台查询
 	return s.selectAccountForModelWithPlatform(ctx, groupID, sessionHash, requestedModel, excludedIDs, platform)
 }
 
@@ -454,7 +445,8 @@ func (s *GatewayService) SelectAccountWithLoadAwareness(ctx context.Context, gro
 		accountID, err := s.cache.GetSessionAccountID(ctx, sessionHash)
 		if err == nil && accountID > 0 && !isExcluded(accountID) {
 			account, err := s.accountRepo.GetByID(ctx, accountID)
-			if err == nil && s.isAccountAllowedForPlatform(account, platform, useMixed) &&
+			if err == nil && s.isAccountInGroup(account, groupID) &&
+				s.isAccountAllowedForPlatform(account, platform, useMixed) &&
 				account.IsSchedulable() &&
 				(requestedModel == "" || s.isModelSupportedByAccount(account, requestedModel)) {
 				result, err := s.tryAcquireAccountSlot(ctx, accountID, account.Concurrency)
@@ -671,9 +663,7 @@ func (s *GatewayService) listSchedulableAccounts(ctx context.Context, groupID *i
 		accounts, err = s.accountRepo.ListSchedulableByPlatform(ctx, platform)
 	} else if groupID != nil {
 		accounts, err = s.accountRepo.ListSchedulableByGroupIDAndPlatform(ctx, *groupID, platform)
-		if err == nil && len(accounts) == 0 && hasForcePlatform {
-			accounts, err = s.accountRepo.ListSchedulableByPlatform(ctx, platform)
-		}
+		// 分组内无账号则返回空列表，由上层处理错误，不再回退到全平台查询
 	} else {
 		accounts, err = s.accountRepo.ListSchedulableByPlatform(ctx, platform)
 	}
@@ -696,6 +686,23 @@ func (s *GatewayService) isAccountAllowedForPlatform(account *Account, platform
 	return account.Platform == platform
 }
 
+// isAccountInGroup checks if the account belongs to the specified group.
+// Returns true if groupID is nil (no group restriction) or account belongs to the group.
+func (s *GatewayService) isAccountInGroup(account *Account, groupID *int64) bool {
+	if groupID == nil {
+		return true // 无分组限制
+	}
+	if account == nil {
+		return false
+	}
+	for _, ag := range account.AccountGroups {
+		if ag.GroupID == *groupID {
+			return true
+		}
+	}
+	return false
+}
+
 func (s *GatewayService) tryAcquireAccountSlot(ctx context.Context, accountID int64, maxConcurrency int) (*AcquireResult, error) {
 	if s.concurrencyService == nil {
 		return &AcquireResult{Acquired: true, ReleaseFunc: func() {}}, nil
@@ -734,8 +741,8 @@ func (s *GatewayService) selectAccountForModelWithPlatform(ctx context.Context,
 		if err == nil && accountID > 0 {
 			if _, excluded := excludedIDs[accountID]; !excluded {
 				account, err := s.accountRepo.GetByID(ctx, accountID)
-				// 检查账号平台是否匹配（确保粘性会话不会跨平台）
-				if err == nil && account.Platform == platform && account.IsSchedulable() && (requestedModel == "" || s.isModelSupportedByAccount(account, requestedModel)) {
+				// 检查账号分组归属和平台匹配（确保粘性会话不会跨分组或跨平台）
+				if err == nil && s.isAccountInGroup(account, groupID) && account.Platform == platform && account.IsSchedulable() && (requestedModel == "" || s.isModelSupportedByAccount(account, requestedModel)) {
 					if err := s.cache.RefreshSessionTTL(ctx, sessionHash, stickySessionTTL); err != nil {
 						log.Printf("refresh session ttl failed: session=%s err=%v", sessionHash, err)
 					}
@@ -823,8 +830,8 @@ func (s *GatewayService) selectAccountWithMixedScheduling(ctx context.Context, g
 		if err == nil && accountID > 0 {
 			if _, excluded := excludedIDs[accountID]; !excluded {
 				account, err := s.accountRepo.GetByID(ctx, accountID)
-				// 检查账号是否有效：原生平台直接匹配，antigravity 需要启用混合调度
-				if err == nil && account.IsSchedulable() && (requestedModel == "" || s.isModelSupportedByAccount(account, requestedModel)) {
+				// 检查账号分组归属和有效性：原生平台直接匹配，antigravity 需要启用混合调度
+				if err == nil && s.isAccountInGroup(account, groupID) && account.IsSchedulable() && (requestedModel == "" || s.isModelSupportedByAccount(account, requestedModel)) {
 					if account.Platform == nativePlatform || (account.Platform == PlatformAntigravity && account.IsMixedSchedulingEnabled()) {
 						if err := s.cache.RefreshSessionTTL(ctx, sessionHash, stickySessionTTL); err != nil {
 							log.Printf("refresh session ttl failed: session=%s err=%v", sessionHash, err)