feat(channels): add custom account stats pricing rules

Allow channels to configure independent model pricing for account
statistics cost calculation, decoupled from user billing.

Backend:
- Migration 101: channels.apply_pricing_to_account_stats toggle,
  channel_account_stats_pricing_rules/model_pricing tables,
  usage_logs.account_stats_cost column
- resolveAccountStatsCost: match rules by group/account, then channel
  pricing, fallback to original formula when unconfigured
- Integrate into both GatewayService.recordUsageCore and
  OpenAIGatewayService.RecordUsage
- Update 8 account stats SQL queries to use
  COALESCE(account_stats_cost, total_cost) * account_rate_multiplier
- 23 unit tests for matching, pricing lookup, and cost calculation

Frontend:
- Channel edit dialog: toggle + custom rules UI with group/account
  multi-select and pricing entry cards
- API types and i18n (zh/en)

backend/internal/service/channel.go

@@ -49,21 +49,25 @@ type Channel struct {
 	ModelPricing []ChannelModelPricing
 	// 渠道级模型映射（按平台分组：platform → {src→dst}）
 	ModelMapping map[string]map[string]string
-	// 渠道特性配置（如 {"web_search_emulation": {"anthropic": true}}）
-	FeaturesConfig map[string]any
+
+	// 账号统计定价
+	ApplyPricingToAccountStats bool                      // 是否应用渠道模型定价到账号统计
+	AccountStatsPricingRules   []AccountStatsPricingRule // 自定义账号统计定价规则（按 SortOrder 排序，先命中为准）
 }
 
-// IsWebSearchEmulationEnabled 返回该渠道是否为指定平台启用了 web search 模拟。
-func (c *Channel) IsWebSearchEmulationEnabled(platform string) bool {
-	if c == nil || c.FeaturesConfig == nil {
-		return false
-	}
-	wse, ok := c.FeaturesConfig[featureKeyWebSearchEmulation].(map[string]any)
-	if !ok {
-		return false
-	}
-	enabled, ok := wse[platform].(bool)
-	return ok && enabled
+// AccountStatsPricingRule 账号统计定价规则
+// 每条规则包含匹配条件（分组/账号）和独立的模型定价。
+// 多条规则按 SortOrder 排序，先命中为准。
+type AccountStatsPricingRule struct {
+	ID         int64
+	ChannelID  int64
+	Name       string
+	GroupIDs   []int64
+	AccountIDs []int64
+	SortOrder  int
+	Pricing    []ChannelModelPricing // 规则内的模型定价（复用现有定价结构）
+	CreatedAt  time.Time
+	UpdatedAt  time.Time
 }
 
 // ChannelModelPricing 渠道模型定价条目
@@ -192,6 +196,26 @@ func (c *Channel) Clone() *Channel {
 			cp.ModelMapping[platform] = inner
 		}
 	}
+	if c.AccountStatsPricingRules != nil {
+		cp.AccountStatsPricingRules = make([]AccountStatsPricingRule, len(c.AccountStatsPricingRules))
+		for i, rule := range c.AccountStatsPricingRules {
+			cp.AccountStatsPricingRules[i] = rule
+			if rule.GroupIDs != nil {
+				cp.AccountStatsPricingRules[i].GroupIDs = make([]int64, len(rule.GroupIDs))
+				copy(cp.AccountStatsPricingRules[i].GroupIDs, rule.GroupIDs)
+			}
+			if rule.AccountIDs != nil {
+				cp.AccountStatsPricingRules[i].AccountIDs = make([]int64, len(rule.AccountIDs))
+				copy(cp.AccountStatsPricingRules[i].AccountIDs, rule.AccountIDs)
+			}
+			if rule.Pricing != nil {
+				cp.AccountStatsPricingRules[i].Pricing = make([]ChannelModelPricing, len(rule.Pricing))
+				for j := range rule.Pricing {
+					cp.AccountStatsPricingRules[i].Pricing[j] = rule.Pricing[j].Clone()
+				}
+			}
+		}
+	}
 	return &cp
 }