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)

frontend/src/i18n/locales/en.ts

@@ -1844,7 +1844,18 @@ export default {
         noPlatforms: 'Click "Add Platform" to start configuring the channel',
         mappingCount: 'mappings',
         pricingEntry: 'Pricing Entry',
-        noModels: 'No models added'
+        noModels: 'No models added',
+        applyPricingToAccountStats: 'Apply Pricing to Account Stats',
+        applyPricingToAccountStatsDesc: 'When enabled, account statistics cost will use channel model pricing. Account rate multiplier still applies.',
+        accountStatsPricingRules: 'Custom Account Stats Pricing Rules',
+        addRule: 'Add Rule',
+        noRulesConfigured: 'No custom rules configured. Channel model pricing above will be used.',
+        ruleName: 'Rule name (optional)',
+        ruleGroups: 'Groups',
+        ruleAccounts: 'Account IDs',
+        ruleAccountsPlaceholder: 'Enter account IDs, comma-separated',
+        ruleModelPricing: 'Model Pricing',
+        noGroupsInChannel: 'No groups selected in platform tabs above'
       }
     },
 

frontend/src/i18n/locales/zh.ts

@@ -1923,7 +1923,18 @@ export default {
         noPlatforms: '点击"添加平台"开始配置渠道',
         mappingCount: '条映射',
         pricingEntry: '定价配置',
-        noModels: '未添加模型'
+        noModels: '未添加模型',
+        applyPricingToAccountStats: '应用模型定价到账号统计',
+        applyPricingToAccountStatsDesc: '启用后，账号统计费用将使用渠道模型定价计算。账号自身的统计倍率仍然生效。',
+        accountStatsPricingRules: '自定义账号统计定价规则',
+        addRule: '添加规则',
+        noRulesConfigured: '未配置自定义规则，将使用上方的模型定价。',
+        ruleName: '规则名称（可选）',
+        ruleGroups: '分组',
+        ruleAccounts: '账号 ID',
+        ruleAccountsPlaceholder: '输入账号 ID，逗号分隔',
+        ruleModelPricing: '模型定价',
+        noGroupsInChannel: '上方平台标签页中未选择分组'
       }
     },