feat(计费): 支持账号计费倍率快照与统计展示

- 新增 accounts.rate_multiplier（默认 1.0，允许 0）
- 使用 usage_logs.account_rate_multiplier 记录倍率快照，避免历史回算
- 统计/导出/管理端展示账号口径费用（total_cost * account_rate_multiplier）

frontend/src/components/account/CreateAccountModal.vue

@@ -1196,7 +1196,7 @@
         <ProxySelector v-model="form.proxy_id" :proxies="proxies" />
       </div>
 
-      <div class="grid grid-cols-2 gap-4">
+      <div class="grid grid-cols-2 gap-4 lg:grid-cols-3">
         <div>
           <label class="input-label">{{ t('admin.accounts.concurrency') }}</label>
           <input v-model.number="form.concurrency" type="number" min="1" class="input" />
@@ -1212,6 +1212,11 @@
           />
           <p class="input-hint">{{ t('admin.accounts.priorityHint') }}</p>
         </div>
+        <div>
+          <label class="input-label">{{ t('admin.accounts.billingRateMultiplier') }}</label>
+          <input v-model.number="form.rate_multiplier" type="number" min="0" step="0.01" class="input" />
+          <p class="input-hint">{{ t('admin.accounts.billingRateMultiplierHint') }}</p>
+        </div>
       </div>
       <div class="border-t border-gray-200 pt-4 dark:border-dark-600">
         <label class="input-label">{{ t('admin.accounts.expiresAt') }}</label>
@@ -1832,6 +1837,7 @@ const form = reactive({
   proxy_id: null as number | null,
   concurrency: 10,
   priority: 1,
+  rate_multiplier: 1,
   group_ids: [] as number[],
   expires_at: null as number | null
 })
@@ -2119,6 +2125,7 @@ const resetForm = () => {
   form.proxy_id = null
   form.concurrency = 10
   form.priority = 1
+  form.rate_multiplier = 1
   form.group_ids = []
   form.expires_at = null
   accountCategory.value = 'oauth-based'
@@ -2272,6 +2279,7 @@ const createAccountAndFinish = async (
     proxy_id: form.proxy_id,
     concurrency: form.concurrency,
     priority: form.priority,
+    rate_multiplier: form.rate_multiplier,
     group_ids: form.group_ids,
     expires_at: form.expires_at,
     auto_pause_on_expired: autoPauseOnExpired.value
@@ -2490,6 +2498,7 @@ const handleCookieAuth = async (sessionKey: string) => {
           proxy_id: form.proxy_id,
           concurrency: form.concurrency,
           priority: form.priority,
+          rate_multiplier: form.rate_multiplier,
           group_ids: form.group_ids,
           expires_at: form.expires_at,
           auto_pause_on_expired: autoPauseOnExpired.value