fix(channel): use upstream model for account stats pricing and remove channel pricing fallback

- resolveAccountStatsCost now uses the final upstream model (after
  account-level mapping) to match custom pricing rules, fixing the
  issue where requested model (e.g. claude-sonnet-4-5) didn't match
  rules configured for upstream model (e.g. claude-opus-4-6)
- Remove tryChannelPricing fallback — only custom rules are applied,
  unmatched requests use default formula (total_cost × rate)
- Remove unused billingService and serviceTier parameters
- Update description: "启用后将支持自定义账号统计的模型价格"

frontend/src/i18n/locales/en.ts

@@ -1877,7 +1877,7 @@ export default {
         pricingEntry: 'Pricing Entry',
         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.',
+        applyPricingToAccountStatsDesc: 'When enabled, custom account stats model pricing rules will be applied.',
         accountStatsPricingRules: 'Custom Account Stats Pricing Rules',
         addRule: 'Add Rule',
         noRulesConfigured: 'No custom rules configured. Channel model pricing above will be used.',