perf(网关): 复用分组上下文减少热路径查询

新增 GetByIDLite 并在网关与 Gemini 选择流程复用上下文 group，避免 COUNT 触发
更新 API key 中间件注入 group 上下文，减少重复查库
补充 gateway/gemini 中间件与仓库层回归测试

测试: make test

backend/internal/service/gateway_service.go

@@ -361,27 +361,13 @@ func (s *GatewayService) SelectAccountForModelWithExclusions(ctx context.Context
 	if hasForcePlatform && forcePlatform != "" {
 		platform = forcePlatform
 	} else if groupID != nil {
-		// 根据分组 platform 决定查询哪种账号
-		group, err := s.groupRepo.GetByID(ctx, *groupID)
+		group, resolvedGroupID, err := s.resolveGatewayGroup(ctx, groupID)
 		if err != nil {
-			return nil, fmt.Errorf("get group failed: %w", err)
+			return nil, err
 		}
+		groupID = resolvedGroupID
+		ctx = s.withGroupContext(ctx, group)
 		platform = group.Platform
-
-		// 检查 Claude Code 客户端限制
-		if group.ClaudeCodeOnly {
-			isClaudeCode := IsClaudeCodeClient(ctx)
-			if !isClaudeCode {
-				// 非 Claude Code 客户端，检查是否有降级分组
-				if group.FallbackGroupID != nil {
-					// 使用降级分组重新调度
-					fallbackGroupID := *group.FallbackGroupID
-					return s.SelectAccountForModelWithExclusions(ctx, &fallbackGroupID, sessionHash, requestedModel, excludedIDs)
-				}
-				// 无降级分组，拒绝访问
-				return nil, ErrClaudeCodeOnly
-			}
-		}
 	} else {
 		// 无分组时只使用原生 anthropic 平台
 		platform = PlatformAnthropic
@@ -409,10 +395,11 @@ func (s *GatewayService) SelectAccountWithLoadAwareness(ctx context.Context, gro
 	}
 
 	// 检查 Claude Code 客户端限制（可能会替换 groupID 为降级分组）
-	groupID, err := s.checkClaudeCodeRestriction(ctx, groupID)
+	group, groupID, err := s.checkClaudeCodeRestriction(ctx, groupID)
 	if err != nil {
 		return nil, err
 	}
+	ctx = s.withGroupContext(ctx, group)
 
 	if s.concurrencyService == nil || !cfg.LoadBatchEnabled {
 		account, err := s.SelectAccountForModelWithExclusions(ctx, groupID, sessionHash, requestedModel, excludedIDs)
@@ -452,7 +439,7 @@ func (s *GatewayService) SelectAccountWithLoadAwareness(ctx context.Context, gro
 		}, nil
 	}
 
-	platform, hasForcePlatform, err := s.resolvePlatform(ctx, groupID)
+	platform, hasForcePlatform, err := s.resolvePlatform(ctx, groupID, group)
 	if err != nil {
 		return nil, err
 	}
@@ -652,51 +639,91 @@ func (s *GatewayService) schedulingConfig() config.GatewaySchedulingConfig {
 	}
 }
 
+func (s *GatewayService) withGroupContext(ctx context.Context, group *Group) context.Context {
+	if group == nil {
+		return ctx
+	}
+	if existing, ok := ctx.Value(ctxkey.Group).(*Group); ok && existing != nil && existing.ID == group.ID {
+		return ctx
+	}
+	return context.WithValue(ctx, ctxkey.Group, group)
+}
+
+func (s *GatewayService) groupFromContext(ctx context.Context, groupID int64) *Group {
+	if group, ok := ctx.Value(ctxkey.Group).(*Group); ok && group != nil && group.ID == groupID {
+		return group
+	}
+	return nil
+}
+
+func (s *GatewayService) resolveGroupByID(ctx context.Context, groupID int64) (*Group, error) {
+	if group := s.groupFromContext(ctx, groupID); group != nil {
+		return group, nil
+	}
+	group, err := s.groupRepo.GetByIDLite(ctx, groupID)
+	if err != nil {
+		return nil, fmt.Errorf("get group failed: %w", err)
+	}
+	return group, nil
+}
+
+func (s *GatewayService) resolveGatewayGroup(ctx context.Context, groupID *int64) (*Group, *int64, error) {
+	if groupID == nil {
+		return nil, nil, nil
+	}
+
+	currentID := *groupID
+	for {
+		group, err := s.resolveGroupByID(ctx, currentID)
+		if err != nil {
+			return nil, nil, err
+		}
+
+		if !group.ClaudeCodeOnly || IsClaudeCodeClient(ctx) {
+			return group, &currentID, nil
+		}
+
+		if group.FallbackGroupID == nil {
+			return nil, nil, ErrClaudeCodeOnly
+		}
+		currentID = *group.FallbackGroupID
+	}
+}
+
 // checkClaudeCodeRestriction 检查分组的 Claude Code 客户端限制
 // 如果分组启用了 claude_code_only 且请求不是来自 Claude Code 客户端：
 //   - 有降级分组：返回降级分组的 ID
 //   - 无降级分组：返回 ErrClaudeCodeOnly 错误
-func (s *GatewayService) checkClaudeCodeRestriction(ctx context.Context, groupID *int64) (*int64, error) {
+func (s *GatewayService) checkClaudeCodeRestriction(ctx context.Context, groupID *int64) (*Group, *int64, error) {
 	if groupID == nil {
-		return groupID, nil
+		return nil, groupID, nil
 	}
 
 	// 强制平台模式不检查 Claude Code 限制
 	if _, hasForcePlatform := ctx.Value(ctxkey.ForcePlatform).(string); hasForcePlatform {
-		return groupID, nil
+		return nil, groupID, nil
 	}
 
-	group, err := s.groupRepo.GetByID(ctx, *groupID)
+	group, resolvedID, err := s.resolveGatewayGroup(ctx, groupID)
 	if err != nil {
-		return nil, fmt.Errorf("get group failed: %w", err)
+		return nil, nil, err
 	}
 
-	if !group.ClaudeCodeOnly {
-		return groupID, nil
-	}
-
-	// 分组启用了 Claude Code 限制
-	if IsClaudeCodeClient(ctx) {
-		return groupID, nil
-	}
-
-	// 非 Claude Code 客户端，检查降级分组
-	if group.FallbackGroupID != nil {
-		return group.FallbackGroupID, nil
-	}
-
-	return nil, ErrClaudeCodeOnly
+	return group, resolvedID, nil
 }
 
-func (s *GatewayService) resolvePlatform(ctx context.Context, groupID *int64) (string, bool, error) {
+func (s *GatewayService) resolvePlatform(ctx context.Context, groupID *int64, group *Group) (string, bool, error) {
 	forcePlatform, hasForcePlatform := ctx.Value(ctxkey.ForcePlatform).(string)
 	if hasForcePlatform && forcePlatform != "" {
 		return forcePlatform, true, nil
 	}
+	if group != nil {
+		return group.Platform, false, nil
+	}
 	if groupID != nil {
-		group, err := s.groupRepo.GetByID(ctx, *groupID)
+		group, err := s.resolveGroupByID(ctx, *groupID)
 		if err != nil {
-			return "", false, fmt.Errorf("get group failed: %w", err)
+			return "", false, err
 		}
 		return group.Platform, false, nil
 	}