feat(queue): 优化用户消息队列锁释放时机

将队列锁释放时机从"请求完成后"提前到"请求发送后"，因为 Claude API
限流（RPM）基于请求发送时刻计算，无需等待响应完成。

主要变更：
- 移除锁续租机制（startLockRenewal、refreshUserMessageLock）
- 所有 relay 服务在请求发送成功后立即释放锁
- 流式请求通过 onResponseStart 回调在收到响应头时释放
- 调整默认配置：timeoutMs 60s→5s，lockTtlMs 120s→5s
- 新增 USER_MESSAGE_QUEUE_LOCK_TTL_MS 环境变量支持

tests/userMessageQueue.test.js

@@ -179,88 +179,6 @@ describe('UserMessageQueueService', () => {
     })
   })
 
-  describe('startLockRenewal', () => {
-    beforeEach(() => {
-      jest.useFakeTimers()
-    })
-
-    afterEach(() => {
-      jest.useRealTimers()
-      jest.restoreAllMocks()
-    })
-
-    it('should periodically refresh lock while enabled', async () => {
-      jest.spyOn(userMessageQueueService, 'getConfig').mockResolvedValue({
-        enabled: true,
-        delayMs: 200,
-        timeoutMs: 30000,
-        lockTtlMs: 120000
-      })
-      const refreshSpy = jest.spyOn(redis, 'refreshUserMessageLock').mockResolvedValue(true)
-
-      const stop = await userMessageQueueService.startLockRenewal('acct-1', 'req-1')
-
-      jest.advanceTimersByTime(60000) // 半个TTL
-      await Promise.resolve()
-
-      expect(refreshSpy).toHaveBeenCalledWith('acct-1', 'req-1', 120000)
-
-      stop()
-    })
-
-    it('should no-op when queue disabled', async () => {
-      jest.spyOn(userMessageQueueService, 'getConfig').mockResolvedValue({
-        enabled: false,
-        delayMs: 200,
-        timeoutMs: 30000,
-        lockTtlMs: 120000
-      })
-      const refreshSpy = jest.spyOn(redis, 'refreshUserMessageLock').mockResolvedValue(true)
-
-      const stop = await userMessageQueueService.startLockRenewal('acct-1', 'req-1')
-      jest.advanceTimersByTime(120000)
-      await Promise.resolve()
-
-      expect(refreshSpy).not.toHaveBeenCalled()
-      stop()
-    })
-
-    it('should track active renewal timer', async () => {
-      jest.spyOn(userMessageQueueService, 'getConfig').mockResolvedValue({
-        enabled: true,
-        delayMs: 200,
-        timeoutMs: 30000,
-        lockTtlMs: 120000
-      })
-      jest.spyOn(redis, 'refreshUserMessageLock').mockResolvedValue(true)
-
-      expect(userMessageQueueService.getActiveRenewalCount()).toBe(0)
-
-      const stop = await userMessageQueueService.startLockRenewal('acct-1', 'req-1')
-      expect(userMessageQueueService.getActiveRenewalCount()).toBe(1)
-
-      stop()
-      expect(userMessageQueueService.getActiveRenewalCount()).toBe(0)
-    })
-
-    it('should stop all renewal timers on service shutdown', async () => {
-      jest.spyOn(userMessageQueueService, 'getConfig').mockResolvedValue({
-        enabled: true,
-        delayMs: 200,
-        timeoutMs: 30000,
-        lockTtlMs: 120000
-      })
-      jest.spyOn(redis, 'refreshUserMessageLock').mockResolvedValue(true)
-
-      await userMessageQueueService.startLockRenewal('acct-1', 'req-1')
-      await userMessageQueueService.startLockRenewal('acct-2', 'req-2')
-      expect(userMessageQueueService.getActiveRenewalCount()).toBe(2)
-
-      userMessageQueueService.stopAllRenewalTimers()
-      expect(userMessageQueueService.getActiveRenewalCount()).toBe(0)
-    })
-  })
-
   describe('acquireQueueLock', () => {
     afterEach(() => {
       jest.restoreAllMocks()