fix: 修复 _email_poller 线程静默死亡问题

- IMAP 连接加 30s socket timeout，防止 _send_id_command 等操作永久阻塞
- _email_poller 加顶层 try/except，异常后 sleep 5s 继续循环
- @retry() 加指数退避延迟(1s,2s,4s...)，避免重试风暴

main.py

@@ -45,13 +45,17 @@ def _signal_handler(signum, frame):
 def _email_poller(cfg):
     logger.info("邮件轮询线程已启动")
     while _running:
-        for acct_idx, mail in poll_accounts(cfg):
-            if not _running:
-                return
-            with _pending_lock:
-                if mail.uid not in _pending_uids:
-                    _pending_uids.add(mail.uid)
-                    _email_queue.put((acct_idx, mail))
+        try:
+            for acct_idx, mail in poll_accounts(cfg):
+                if not _running:
+                    return
+                with _pending_lock:
+                    if mail.uid not in _pending_uids:
+                        _pending_uids.add(mail.uid)
+                        _email_queue.put((acct_idx, mail))
+        except Exception as e:
+            logger.error(f"邮件轮询线程异常: {e}", exc_info=True)
+            time.sleep(5)
         if _running:
             for _ in range(cfg.polling.interval_seconds):
                 if not _running: