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

src/email_client.py

@@ -87,12 +87,12 @@ def _select_mailbox(conn, mailbox: str = "INBOX"):
         raise RuntimeError(f"无法选择邮箱 {mailbox}: {data}")
 
 
-def _login_and_prepare(account: EmailAccount):
-    logger.info("连接 %s:%d", account.imap_server, account.imap_port)
+def _login_and_prepare(account: EmailAccount, timeout: int = 30):
+    logger.info("连接 %s:%d (超时%ds)", account.imap_server, account.imap_port, timeout)
     if account.use_ssl:
-        conn = imaplib.IMAP4_SSL(account.imap_server, account.imap_port)
+        conn = imaplib.IMAP4_SSL(account.imap_server, account.imap_port, timeout=timeout)
     else:
-        conn = imaplib.IMAP4(account.imap_server, account.imap_port)
+        conn = imaplib.IMAP4(account.imap_server, account.imap_port, timeout=timeout)
         if account.use_starttls:
             conn.starttls()
     conn.login(account.username, account.password)