refactor: split buildCache into sub-functions, reduce nesting 5→2

- Extract newEmptyChannelCache() factory to deduplicate map init
- Extract expandPricingToCache() for model pricing expansion
- Extract expandMappingToCache() for model mapping expansion
- buildCache reduced from 110 to 50 lines

backend/internal/service/channel_service.go

@@ -183,6 +183,67 @@ func (s *ChannelService) loadCache(ctx context.Context) (*channelCache, error) {
 	return cache, nil
 }
 
+// newEmptyChannelCache 创建空的渠道缓存（所有 map 已初始化）
+func newEmptyChannelCache() *channelCache {
+	return &channelCache{
+		pricingByGroupModel:     make(map[channelModelKey]*ChannelModelPricing),
+		wildcardByGroupPlatform: make(map[channelGroupPlatformKey][]*wildcardPricingEntry),
+		mappingByGroupModel:     make(map[channelModelKey]string),
+		wildcardMappingByGP:     make(map[channelGroupPlatformKey][]*wildcardMappingEntry),
+		channelByGroupID:        make(map[int64]*Channel),
+		groupPlatform:           make(map[int64]string),
+		byID:                    make(map[int64]*Channel),
+	}
+}
+
+// expandPricingToCache 将渠道的模型定价展开到缓存（按分组+平台维度）。
+// antigravity 平台同时服务 Claude 和 Gemini 模型，需匹配 anthropic/gemini 的定价条目。
+func expandPricingToCache(cache *channelCache, ch *Channel, gid int64, platform string) {
+	for j := range ch.ModelPricing {
+		pricing := &ch.ModelPricing[j]
+		if !isPlatformPricingMatch(platform, pricing.Platform) {
+			continue // 跳过非本平台的定价
+		}
+		gpKey := channelGroupPlatformKey{groupID: gid, platform: platform}
+		for _, model := range pricing.Models {
+			if strings.HasSuffix(model, "*") {
+				prefix := strings.ToLower(strings.TrimSuffix(model, "*"))
+				cache.wildcardByGroupPlatform[gpKey] = append(cache.wildcardByGroupPlatform[gpKey], &wildcardPricingEntry{
+					prefix:  prefix,
+					pricing: pricing,
+				})
+			} else {
+				key := channelModelKey{groupID: gid, platform: platform, model: strings.ToLower(model)}
+				cache.pricingByGroupModel[key] = pricing
+			}
+		}
+	}
+}
+
+// expandMappingToCache 将渠道的模型映射展开到缓存（按分组+平台维度）。
+// antigravity 平台同时服务 Claude 和 Gemini 模型。
+func expandMappingToCache(cache *channelCache, ch *Channel, gid int64, platform string) {
+	for _, mappingPlatform := range matchingPlatforms(platform) {
+		platformMapping, ok := ch.ModelMapping[mappingPlatform]
+		if !ok {
+			continue
+		}
+		gpKey := channelGroupPlatformKey{groupID: gid, platform: platform}
+		for src, dst := range platformMapping {
+			if strings.HasSuffix(src, "*") {
+				prefix := strings.ToLower(strings.TrimSuffix(src, "*"))
+				cache.wildcardMappingByGP[gpKey] = append(cache.wildcardMappingByGP[gpKey], &wildcardMappingEntry{
+					prefix: prefix,
+					target: dst,
+				})
+			} else {
+				key := channelModelKey{groupID: gid, platform: platform, model: strings.ToLower(src)}
+				cache.mappingByGroupModel[key] = dst
+			}
+		}
+	}
+}
+
 // buildCache 从数据库构建渠道缓存。
 // 使用独立 context 避免请求取消导致空值被长期缓存。
 func (s *ChannelService) buildCache(ctx context.Context) (*channelCache, error) {
@@ -194,16 +255,8 @@ func (s *ChannelService) buildCache(ctx context.Context) (*channelCache, error)
 	if err != nil {
 		// error-TTL：失败时存入短 TTL 空缓存，防止紧密重试
 		slog.Warn("failed to build channel cache", "error", err)
-		errorCache := &channelCache{
-			pricingByGroupModel:     make(map[channelModelKey]*ChannelModelPricing),
-			wildcardByGroupPlatform: make(map[channelGroupPlatformKey][]*wildcardPricingEntry),
-			mappingByGroupModel:     make(map[channelModelKey]string),
-			wildcardMappingByGP:     make(map[channelGroupPlatformKey][]*wildcardMappingEntry),
-			channelByGroupID:        make(map[int64]*Channel),
-			groupPlatform:           make(map[int64]string),
-			byID:                    make(map[int64]*Channel),
-			loadedAt:                time.Now().Add(-(channelCacheTTL - channelErrorTTL)), // 使剩余 TTL = errorTTL
-		}
+		errorCache := newEmptyChannelCache()
+		errorCache.loadedAt = time.Now().Add(-(channelCacheTTL - channelErrorTTL)) // 使剩余 TTL = errorTTL
 		s.cache.Store(errorCache)
 		return nil, fmt.Errorf("list all channels: %w", err)
 	}
@@ -222,71 +275,20 @@ func (s *ChannelService) buildCache(ctx context.Context) (*channelCache, error)
 		}
 	}
 
-	cache := &channelCache{
-		pricingByGroupModel:     make(map[channelModelKey]*ChannelModelPricing),
-		wildcardByGroupPlatform: make(map[channelGroupPlatformKey][]*wildcardPricingEntry),
-		mappingByGroupModel:     make(map[channelModelKey]string),
-		wildcardMappingByGP:     make(map[channelGroupPlatformKey][]*wildcardMappingEntry),
-		channelByGroupID:        make(map[int64]*Channel),
-		groupPlatform:           groupPlatforms,
-		byID:                    make(map[int64]*Channel, len(channels)),
-		loadedAt:                time.Now(),
-	}
+	cache := newEmptyChannelCache()
+	cache.groupPlatform = groupPlatforms
+	cache.byID = make(map[int64]*Channel, len(channels))
+	cache.loadedAt = time.Now()
 
 	for i := range channels {
 		ch := &channels[i]
 		cache.byID[ch.ID] = ch
 
-		// 展开到分组维度
 		for _, gid := range ch.GroupIDs {
 			cache.channelByGroupID[gid] = ch
-			platform := groupPlatforms[gid] // e.g. "anthropic"
-
-			// 只展开该平台的模型定价到 (groupID, platform, model) → *ChannelModelPricing
-			// antigravity 平台同时服务 Claude 和 Gemini 模型，需匹配 anthropic/gemini 的定价条目
-			for j := range ch.ModelPricing {
-				pricing := &ch.ModelPricing[j]
-				if !isPlatformPricingMatch(platform, pricing.Platform) {
-					continue // 跳过非本平台的定价
-				}
-				for _, model := range pricing.Models {
-					if strings.HasSuffix(model, "*") {
-						// 通配符模型 → 存入 wildcardByGroupPlatform
-						prefix := strings.ToLower(strings.TrimSuffix(model, "*"))
-						gpKey := channelGroupPlatformKey{groupID: gid, platform: platform}
-						cache.wildcardByGroupPlatform[gpKey] = append(cache.wildcardByGroupPlatform[gpKey], &wildcardPricingEntry{
-							prefix:  prefix,
-							pricing: pricing,
-						})
-					} else {
-						key := channelModelKey{groupID: gid, platform: platform, model: strings.ToLower(model)}
-						cache.pricingByGroupModel[key] = pricing
-					}
-				}
-			}
-
-			// 只展开该平台的模型映射到 (groupID, platform, model) → target
-			// antigravity 平台同时服务 Claude 和 Gemini 模型
-			for _, mappingPlatform := range matchingPlatforms(platform) {
-				platformMapping, ok := ch.ModelMapping[mappingPlatform]
-				if !ok {
-					continue
-				}
-				for src, dst := range platformMapping {
-					if strings.HasSuffix(src, "*") {
-						// 通配符映射 → 存入 wildcardMappingByGP
-						prefix := strings.ToLower(strings.TrimSuffix(src, "*"))
-						gpKey := channelGroupPlatformKey{groupID: gid, platform: platform}
-						cache.wildcardMappingByGP[gpKey] = append(cache.wildcardMappingByGP[gpKey], &wildcardMappingEntry{
-							prefix: prefix,
-							target: dst,
-						})
-					} else {
-						key := channelModelKey{groupID: gid, platform: platform, model: strings.ToLower(src)}
-						cache.mappingByGroupModel[key] = dst
-					}
-				}
-			}
+			platform := groupPlatforms[gid]
+			expandPricingToCache(cache, ch, gid, platform)
+			expandMappingToCache(cache, ch, gid, platform)
 		}
 	}
 
@@ -362,26 +364,48 @@ func (s *ChannelService) GetChannelForGroup(ctx context.Context, groupID int64)
 	return ch.Clone(), nil
 }
 
+// channelLookup 热路径公共查找结果
+type channelLookup struct {
+	cache    *channelCache
+	channel  *Channel
+	platform string
+}
+
+// lookupGroupChannel 加载缓存并查找分组对应的渠道信息（公共热路径前置逻辑）。
+// 返回 nil 且 err==nil 表示分组无活跃渠道；err!=nil 表示缓存加载失败。
+func (s *ChannelService) lookupGroupChannel(ctx context.Context, groupID int64) (*channelLookup, error) {
+	cache, err := s.loadCache(ctx)
+	if err != nil {
+		return nil, err
+	}
+	ch, ok := cache.channelByGroupID[groupID]
+	if !ok || !ch.IsActive() {
+		return nil, nil
+	}
+	return &channelLookup{
+		cache:    cache,
+		channel:  ch,
+		platform: cache.groupPlatform[groupID],
+	}, nil
+}
+
 // GetChannelModelPricing 获取指定分组+模型的渠道定价（热路径 O(1)）
 func (s *ChannelService) GetChannelModelPricing(ctx context.Context, groupID int64, model string) *ChannelModelPricing {
-	cache, err := s.loadCache(ctx)
+	lk, err := s.lookupGroupChannel(ctx, groupID)
 	if err != nil {
 		slog.Warn("failed to load channel cache", "group_id", groupID, "error", err)
 		return nil
 	}
-
-	// 检查渠道是否启用
-	ch, ok := cache.channelByGroupID[groupID]
-	if !ok || !ch.IsActive() {
+	if lk == nil {
 		return nil
 	}
 
-	platform := cache.groupPlatform[groupID]
-	key := channelModelKey{groupID: groupID, platform: platform, model: strings.ToLower(model)}
-	pricing, ok := cache.pricingByGroupModel[key]
+	modelLower := strings.ToLower(model)
+	key := channelModelKey{groupID: groupID, platform: lk.platform, model: modelLower}
+	pricing, ok := lk.cache.pricingByGroupModel[key]
 	if !ok {
 		// 精确查找失败，尝试通配符匹配
-		pricing = cache.matchWildcard(groupID, platform, strings.ToLower(model))
+		pricing = lk.cache.matchWildcard(groupID, lk.platform, modelLower)
 		if pricing == nil {
 			return nil
 		}
@@ -394,31 +418,57 @@ func (s *ChannelService) GetChannelModelPricing(ctx context.Context, groupID int
 // ResolveChannelMapping 解析渠道级模型映射（热路径 O(1)）
 // 返回映射结果，包含映射后的模型名、渠道 ID、计费模型来源。
 func (s *ChannelService) ResolveChannelMapping(ctx context.Context, groupID int64, model string) ChannelMappingResult {
-	cache, err := s.loadCache(ctx)
-	if err != nil {
+	lk, _ := s.lookupGroupChannel(ctx, groupID)
+	if lk == nil {
 		return ChannelMappingResult{MappedModel: model}
 	}
+	return resolveMapping(lk, groupID, model)
+}
 
-	ch, ok := cache.channelByGroupID[groupID]
-	if !ok || !ch.IsActive() {
-		return ChannelMappingResult{MappedModel: model}
+// IsModelRestricted 检查模型是否被渠道限制。
+// 返回 true 表示模型被限制（不在允许列表中）。
+// 如果渠道未启用模型限制或分组无渠道关联，返回 false。
+func (s *ChannelService) IsModelRestricted(ctx context.Context, groupID int64, model string) bool {
+	lk, _ := s.lookupGroupChannel(ctx, groupID)
+	if lk == nil {
+		return false
 	}
+	return checkRestricted(lk, groupID, model)
+}
 
+// ResolveChannelMappingAndRestrict 解析渠道映射并检查模型限制（组合方法）。
+// 返回映射结果和是否被限制。groupID 为 nil 时跳过。
+func (s *ChannelService) ResolveChannelMappingAndRestrict(ctx context.Context, groupID *int64, model string) (ChannelMappingResult, bool) {
+	if groupID == nil {
+		return ChannelMappingResult{MappedModel: model}, false
+	}
+	lk, _ := s.lookupGroupChannel(ctx, *groupID)
+	if lk == nil {
+		return ChannelMappingResult{MappedModel: model}, false
+	}
+	// 先用原始模型检查定价列表限制，再做映射
+	restricted := checkRestricted(lk, *groupID, model)
+	mapping := resolveMapping(lk, *groupID, model)
+	return mapping, restricted
+}
+
+// resolveMapping 基于已查找的渠道信息解析模型映射
+func resolveMapping(lk *channelLookup, groupID int64, model string) ChannelMappingResult {
 	result := ChannelMappingResult{
 		MappedModel:        model,
-		ChannelID:          ch.ID,
-		BillingModelSource: ch.BillingModelSource,
+		ChannelID:          lk.channel.ID,
+		BillingModelSource: lk.channel.BillingModelSource,
 	}
 	if result.BillingModelSource == "" {
 		result.BillingModelSource = BillingModelSourceChannelMapped
 	}
 
-	platform := cache.groupPlatform[groupID]
-	key := channelModelKey{groupID: groupID, platform: platform, model: strings.ToLower(model)}
-	if mapped, ok := cache.mappingByGroupModel[key]; ok {
+	modelLower := strings.ToLower(model)
+	key := channelModelKey{groupID: groupID, platform: lk.platform, model: modelLower}
+	if mapped, ok := lk.cache.mappingByGroupModel[key]; ok {
 		result.MappedModel = mapped
 		result.Mapped = true
-	} else if mapped := cache.matchWildcardMapping(groupID, platform, strings.ToLower(model)); mapped != "" {
+	} else if mapped := lk.cache.matchWildcardMapping(groupID, lk.platform, modelLower); mapped != "" {
 		result.MappedModel = mapped
 		result.Mapped = true
 	}
@@ -426,48 +476,24 @@ func (s *ChannelService) ResolveChannelMapping(ctx context.Context, groupID int6
 	return result
 }
 
-// IsModelRestricted 检查模型是否被渠道限制。
-// 返回 true 表示模型被限制（不在允许列表中）。
-// 如果渠道未启用模型限制或分组无渠道关联，返回 false。
-func (s *ChannelService) IsModelRestricted(ctx context.Context, groupID int64, model string) bool {
-	cache, err := s.loadCache(ctx)
-	if err != nil {
-		return false // 缓存加载失败时不限制
-	}
-
-	ch, ok := cache.channelByGroupID[groupID]
-	if !ok || !ch.IsActive() || !ch.RestrictModels {
+// checkRestricted 基于已查找的渠道信息检查模型是否被限制
+func checkRestricted(lk *channelLookup, groupID int64, model string) bool {
+	if !lk.channel.RestrictModels {
 		return false
 	}
-
 	// 检查模型是否在定价列表中
-	platform := cache.groupPlatform[groupID]
-	key := channelModelKey{groupID: groupID, platform: platform, model: strings.ToLower(model)}
-	_, exists := cache.pricingByGroupModel[key]
-	if exists {
+	modelLower := strings.ToLower(model)
+	key := channelModelKey{groupID: groupID, platform: lk.platform, model: modelLower}
+	if _, exists := lk.cache.pricingByGroupModel[key]; exists {
 		return false
 	}
 	// 精确查找失败，尝试通配符匹配
-	if cache.matchWildcard(groupID, platform, strings.ToLower(model)) != nil {
+	if lk.cache.matchWildcard(groupID, lk.platform, modelLower) != nil {
 		return false
 	}
 	return true
 }
 
-// ResolveChannelMappingAndRestrict 解析渠道映射并检查模型限制（组合方法）。
-// 返回映射结果和是否被限制。groupID 为 nil 时跳过。
-func (s *ChannelService) ResolveChannelMappingAndRestrict(ctx context.Context, groupID *int64, model string) (ChannelMappingResult, bool) {
-	var mapping ChannelMappingResult
-	mapping.MappedModel = model
-	if groupID == nil {
-		return mapping, false
-	}
-	// 先用原始模型检查定价列表限制，再做映射
-	restricted := s.IsModelRestricted(ctx, *groupID, model)
-	mapping = s.ResolveChannelMapping(ctx, *groupID, model)
-	return mapping, restricted
-}
-
 // ReplaceModelInBody 替换请求体 JSON 中的 model 字段。
 func ReplaceModelInBody(body []byte, newModel string) []byte {
 	if len(body) == 0 {