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/components/admin/channel/PricingEntryCard.vue

@@ -191,13 +191,13 @@
 
         <!-- Image mode -->
         <div v-else-if="entry.billing_mode === 'image'">
-          <!-- Default image price -->
+          <!-- Default image price (per-request, same as per_request mode) -->
           <label class="mt-3 block text-xs font-medium text-gray-500 dark:text-gray-400">
             {{ t('admin.channels.form.defaultImagePrice', '默认图片价格（未命中层级时使用）') }}
             <span class="ml-1 font-normal text-gray-400">$</span>
           </label>
           <div class="mt-1 w-48">
-            <input :value="entry.image_output_price" @input="emitField('image_output_price', ($event.target as HTMLInputElement).value)"
+            <input :value="entry.per_request_price" @input="emitField('per_request_price', ($event.target as HTMLInputElement).value)"
               type="number" step="any" min="0" class="input text-sm" :placeholder="t('admin.channels.form.pricePlaceholder', '默认')" />
           </div>