feat(frontend): display upstream model in usage table and distribution charts

Show upstream model mapping (requested -> upstream) in UsageTable with arrow notation. Add requested/upstream/mapping source toggle to ModelDistributionChart with lazy loading — only fetches data when user switches tab, with per-source cache invalidation on filter changes. Include upstream_model column in Excel export and i18n for en/zh.

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

@@ -25,8 +25,16 @@
           <span class="text-sm text-gray-900 dark:text-white">{{ row.account?.name || '-' }}</span>
         </template>
 
-        <template #cell-model="{ value }">
-          <span class="font-medium text-gray-900 dark:text-white">{{ value }}</span>
+        <template #cell-model="{ row }">
+          <div v-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>
+            <div class="break-all text-gray-500 dark:text-gray-400">
+              <span class="mr-0.5">↳</span>{{ row.upstream_model }}
+            </div>
+          </div>
+          <span v-else class="font-medium text-gray-900 dark:text-white">{{ row.model }}</span>
         </template>
 
         <template #cell-reasoning_effort="{ row }">