fix(usage): preserve zero user multipliers in cost details

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

@@ -497,7 +497,7 @@
           </div>
           <div class="flex items-center justify-between gap-6">
             <span class="text-gray-400">{{ t('usage.rate') }}</span>
-            <span class="font-semibold text-blue-400">{{ formatMultiplier(tooltipData?.rate_multiplier || 1) }}x</span>
+            <span class="font-semibold text-blue-400">{{ formatMultiplier(tooltipData?.rate_multiplier ?? 1) }}x</span>
           </div>
           <div class="flex items-center justify-between gap-6">
             <span class="text-gray-400">{{ t('usage.original') }}</span>

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

@@ -225,6 +225,22 @@ describe('admin UsageTable tooltip', () => {
     expect(wrapper.get('[data-testid="native-compaction-badge"]').text()).toBe('Compaction')
   })
 
+  it.each([
+    [0, '0.0x'],
+    [0.5, '0.50x'],
+    [undefined, '1.00x'],
+  ])('shows the stored user rate %s in cost details', async (rate, expected) => {
+    const wrapper = mount(UsageTable, {
+      props: { data: [{ ...baseImageRow, rate_multiplier: rate }], loading: false, columns: [] },
+      global: { stubs: { DataTable: DataTableStub, EmptyState: true, Icon: true, Teleport: true } },
+    })
+    const triggers = wrapper.findAll('.group.relative')
+    await triggers[triggers.length - 1].trigger('mouseenter')
+    const rateLabel = wrapper.get('.fixed').findAll('span').find(span => span.text() === 'Rate')!
+    expect(rateLabel.element.parentElement?.textContent).toContain(expected)
+    wrapper.unmount()
+  })
+
   it('shows service tier and billing breakdown in cost tooltip', async () => {
     const row = {
       request_id: 'req-admin-1',