fix(billing): 按账号控制 OpenAI 长上下文计费

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

@@ -168,6 +168,11 @@
           <div class="text-sm">
             <div class="flex items-center gap-1.5">
               <span class="font-medium text-green-600 dark:text-green-400">${{ row.actual_cost?.toFixed(6) || '0.000000' }}</span>
+              <span
+                v-if="row.long_context_billing_applied"
+                data-testid="long-context-billing-marker"
+                class="inline-flex items-center rounded px-1 py-px text-[10px] font-semibold leading-tight bg-amber-100 text-amber-700 ring-1 ring-inset ring-amber-200 dark:bg-amber-500/20 dark:text-amber-300 dark:ring-amber-500/30"
+              >x2</span>
               <!-- Cost Detail Tooltip -->
               <div
                 class="group relative"

frontend/src/components/admin/usage/__tests__/UsageTable.spec.ts

@@ -120,6 +120,38 @@ describe('admin UsageTable tooltip', () => {
     } as DOMRect)
   })
 
+  it('marks only usage rows that actually applied long-context billing', () => {
+    const wrapper = mount(UsageTable, {
+      props: {
+        data: [
+          {
+            ...baseImageRow,
+            request_id: 'req-long-context-enabled',
+            long_context_billing_applied: true,
+          },
+          {
+            ...baseImageRow,
+            request_id: 'req-long-context-disabled',
+            long_context_billing_applied: false,
+          },
+        ],
+        loading: false,
+        columns: [],
+      },
+      global: {
+        stubs: {
+          DataTable: DataTableStub,
+          EmptyState: true,
+          Icon: true,
+          Teleport: true,
+        },
+      },
+    })
+
+    expect(wrapper.findAll('[data-testid="long-context-billing-marker"]')).toHaveLength(1)
+    expect(wrapper.get('[data-testid="long-context-billing-marker"]').text()).toBe('x2')
+  })
+
   it('shows service tier and billing breakdown in cost tooltip', async () => {
     const row = {
       request_id: 'req-admin-1',