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/usage_log.go

@@ -146,6 +146,8 @@ type UsageLog struct {
 	RateMultiplier    float64
 	// AccountRateMultiplier 账号计费倍率快照（nil 表示历史数据，按 1.0 处理）
 	AccountRateMultiplier *float64
+	// AccountStatsCost 账号统计定价预计算费用（nil = 使用默认公式 total_cost × account_rate_multiplier）
+	AccountStatsCost *float64
 
 	BillingType  int8
 	RequestType  RequestType