fix: address audit findings - cache sync, validation, consistency

- clearCreditsExhausted: sync Redis scheduler cache after DB update
- Image billing mode UI: write to per_request_price instead of image_output_price
- OpenAI RecordUsage: use BillingModelSourceRequested constant, add s.cfg nil guard
- Fix i18n key path: admin.channels.perRequestPriceRequired → admin.channels.form.perRequestPriceRequired

frontend/src/views/admin/ChannelsView.vue

@@ -916,7 +916,7 @@ async function handleSubmit() {
       if ((entry.billing_mode === 'per_request' || entry.billing_mode === 'image') &&
           (entry.per_request_price == null || entry.per_request_price === '') &&
           (!entry.intervals || entry.intervals.length === 0)) {
-        appStore.showError(t('admin.channels.perRequestPriceRequired', '按次/图片计费模式必须设置默认价格或至少一个计费层级'))
+        appStore.showError(t('admin.channels.form.perRequestPriceRequired', '按次/图片计费模式必须设置默认价格或至少一个计费层级'))
         return
       }
     }