feat(api): add Claude OAuth usage endpoint with async loading

Add dedicated API endpoint to fetch Claude account OAuth usage data
asynchronously, improving user experience by eliminating the need for
multiple page refreshes to view session window statistics.

Backend changes:
- Add GET /admin/claude-accounts/usage endpoint for batch fetching
- Implement fetchOAuthUsage() to call Claude API /api/oauth/usage
- Add buildClaudeUsageSnapshot() to construct frontend data structure
- Add updateClaudeUsageSnapshot() to persist data to Redis
- Add _toNumberOrNull() helper for safe type conversion
- Update getAllAccounts() to return claudeUsage from Redis cache

Data structure:
- Store three window types: 5h, 7d, 7d-Opus
- Track utilization percentage and reset timestamps
- Calculate remaining seconds for each window

Performance optimizations:
- Concurrent batch requests using Promise.allSettled
- Graceful error handling per account
- Non-blocking async execution

src/routes/admin.js

@@ -2103,6 +2103,61 @@ router.get('/claude-accounts', authenticateAdmin, async (req, res) => {
   }
 })
 
+// 批量获取 Claude 账户的 OAuth Usage 数据
+router.get('/claude-accounts/usage', authenticateAdmin, async (req, res) => {
+  try {
+    const accounts = await redis.getAllClaudeAccounts()
+
+    // 批量并发获取所有活跃 OAuth 账户的 Usage
+    const usagePromises = accounts.map(async (account) => {
+      // 检查是否为 OAuth 账户：scopes 包含 OAuth 相关权限
+      const scopes = account.scopes && account.scopes.trim() ? account.scopes.split(' ') : []
+      const isOAuth = scopes.includes('user:profile') && scopes.includes('user:inference')
+
+      // 仅为 OAuth 授权的活跃账户调用 usage API
+      if (
+        isOAuth &&
+        account.isActive === 'true' &&
+        account.accessToken &&
+        account.status === 'active'
+      ) {
+        try {
+          const usageData = await claudeAccountService.fetchOAuthUsage(account.id)
+          if (usageData) {
+            await claudeAccountService.updateClaudeUsageSnapshot(account.id, usageData)
+          }
+          // 重新读取更新后的数据
+          const updatedAccount = await redis.getClaudeAccount(account.id)
+          return {
+            accountId: account.id,
+            claudeUsage: claudeAccountService.buildClaudeUsageSnapshot(updatedAccount)
+          }
+        } catch (error) {
+          logger.debug(`Failed to fetch OAuth usage for ${account.id}:`, error.message)
+          return { accountId: account.id, claudeUsage: null }
+        }
+      }
+      // Setup Token 账户不调用 usage API，直接返回 null
+      return { accountId: account.id, claudeUsage: null }
+    })
+
+    const results = await Promise.allSettled(usagePromises)
+
+    // 转换为 { accountId: usage } 映射
+    const usageMap = {}
+    results.forEach((result) => {
+      if (result.status === 'fulfilled' && result.value) {
+        usageMap[result.value.accountId] = result.value.claudeUsage
+      }
+    })
+
+    res.json({ success: true, data: usageMap })
+  } catch (error) {
+    logger.error('❌ Failed to fetch Claude accounts usage:', error)
+    res.status(500).json({ error: 'Failed to fetch usage data', message: error.message })
+  }
+})
+
 // 创建新的Claude账户
 router.post('/claude-accounts', authenticateAdmin, async (req, res) => {
   try {