feat(ui): display three-level model mapping chain in usage logs

- Show channel + account mapping steps using model_mapping_chain field
- Add model_mapping_chain to AdminUsageLog TypeScript type
- Fallback to two-level display when chain is not available
- Fix cost nil guard in Anthropic/Antigravity RecordUsage paths
- Bump version to 0.1.105.31

frontend/src/components/admin/usage/UsageTable.vue

@@ -26,7 +26,15 @@
         </template>
 
         <template #cell-model="{ row }">
-          <div v-if="row.upstream_model && row.upstream_model !== row.model" class="space-y-0.5 text-xs">
+          <div v-if="row.model_mapping_chain && row.model_mapping_chain.includes('→')" class="space-y-0.5 text-xs">
+            <div v-for="(step, i) in row.model_mapping_chain.split('→')" :key="i"
+                 class="break-all"
+                 :class="i === 0 ? 'font-medium text-gray-900 dark:text-white' : 'text-gray-500 dark:text-gray-400'"
+                 :style="i > 0 ? `padding-left: ${i * 0.75}rem` : ''">
+              <span v-if="i > 0" class="mr-0.5">↳</span>{{ step }}
+            </div>
+          </div>
+          <div v-else-if="row.upstream_model && row.upstream_model !== row.model" class="space-y-0.5 text-xs">
             <div class="break-all font-medium text-gray-900 dark:text-white">
               {{ row.model }}
             </div>

frontend/src/types/index.ts

@@ -1054,6 +1054,7 @@ export interface UsageLogAccountSummary {
 
 export interface AdminUsageLog extends UsageLog {
   upstream_model?: string | null
+  model_mapping_chain?: string | null
 
   // 账号计费倍率（仅管理员可见）
   account_rate_multiplier?: number | null