fix: 修复PR #458中的totalCostLimit功能问题

主要修复：
- 移除重复的totalUsageLimit字段，统一使用totalCostLimit
- 删除auth.js中重复的总费用限制检查逻辑
- 删除admin.js中重复的totalCostLimit验证代码
- 更新所有前端组件，移除totalUsageLimit引用

功能改进：
- 确保totalCostLimit作为永久累计费用限制正常工作
- 与dailyCostLimit（每日重置）功能互补
- 适用于预付费、一次性API Key场景

测试：
- 删除有逻辑错误的test-total-usage-limit.js
- 创建新的test-total-cost-limit.js验证功能正确性
- 所有测试通过，功能正常工作

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/routes/admin.js

@@ -550,7 +550,6 @@ router.post('/api-keys', authenticateAdmin, async (req, res) => {
       enableClientRestriction,
       allowedClients,
       dailyCostLimit,
-      totalUsageLimit,
       totalCostLimit,
       weeklyOpusCostLimit,
       tags,
@@ -634,22 +633,6 @@ router.post('/api-keys', authenticateAdmin, async (req, res) => {
       return res.status(400).json({ error: 'All tags must be non-empty strings' })
     }
 
-    if (totalUsageLimit !== undefined && totalUsageLimit !== null && totalUsageLimit !== '') {
-      const usageLimit = Number(totalUsageLimit)
-      if (Number.isNaN(usageLimit) || usageLimit < 0) {
-        return res.status(400).json({ error: 'Total usage limit must be a non-negative number' })
-      }
-    }
-
-    if (
-      totalCostLimit !== undefined &&
-      totalCostLimit !== null &&
-      totalCostLimit !== '' &&
-      (Number.isNaN(Number(totalCostLimit)) || Number(totalCostLimit) < 0)
-    ) {
-      return res.status(400).json({ error: 'Total cost limit must be a non-negative number' })
-    }
-
     if (
       totalCostLimit !== undefined &&
       totalCostLimit !== null &&
@@ -704,7 +687,6 @@ router.post('/api-keys', authenticateAdmin, async (req, res) => {
       enableClientRestriction,
       allowedClients,
       dailyCostLimit,
-      totalUsageLimit,
       totalCostLimit,
       weeklyOpusCostLimit,
       tags,
@@ -745,7 +727,6 @@ router.post('/api-keys/batch', authenticateAdmin, async (req, res) => {
       enableClientRestriction,
       allowedClients,
       dailyCostLimit,
-      totalUsageLimit,
       totalCostLimit,
       weeklyOpusCostLimit,
       tags,
@@ -796,7 +777,6 @@ router.post('/api-keys/batch', authenticateAdmin, async (req, res) => {
           enableClientRestriction,
           allowedClients,
           dailyCostLimit,
-          totalUsageLimit,
           totalCostLimit,
           weeklyOpusCostLimit,
           tags,
@@ -915,9 +895,6 @@ router.put('/api-keys/batch', authenticateAdmin, async (req, res) => {
         if (updates.dailyCostLimit !== undefined) {
           finalUpdates.dailyCostLimit = updates.dailyCostLimit
         }
-        if (updates.totalUsageLimit !== undefined) {
-          finalUpdates.totalUsageLimit = updates.totalUsageLimit
-        }
         if (updates.totalCostLimit !== undefined) {
           finalUpdates.totalCostLimit = updates.totalCostLimit
         }
@@ -1049,7 +1026,6 @@ router.put('/api-keys/:keyId', authenticateAdmin, async (req, res) => {
       allowedClients,
       expiresAt,
       dailyCostLimit,
-      totalUsageLimit,
       totalCostLimit,
       weeklyOpusCostLimit,
       tags,
@@ -1208,14 +1184,6 @@ router.put('/api-keys/:keyId', authenticateAdmin, async (req, res) => {
       updates.totalCostLimit = costLimit
     }
 
-    if (totalUsageLimit !== undefined && totalUsageLimit !== null && totalUsageLimit !== '') {
-      const usageLimit = Number(totalUsageLimit)
-      if (Number.isNaN(usageLimit) || usageLimit < 0) {
-        return res.status(400).json({ error: 'Total usage limit must be a non-negative number' })
-      }
-      updates.totalUsageLimit = usageLimit
-    }
-
     // 处理 Opus 周费用限制
     if (
       weeklyOpusCostLimit !== undefined &&