Add network timeouts to prevent container hangs on unreachable servers

Without timeouts, smtplib.SMTP() / imaplib.IMAP4_SSL() / paramiko.Transport()
hang indefinitely when the remote server is down or firewall-dropped, blocking
the entire background thread and eventually freezing the app.

- SMTP: 30s connect/operation timeout
- IMAP: 30s connect/operation timeout
- SFTP (paramiko): 15s socket connect, 15s banner, 30s auth
- SMB and FTP already had timeouts

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/ftp_processor.py

@@ -197,9 +197,19 @@ class _PlainFtpAdapter(_FtpAdapter):
 # SFTP (paramiko)
 # ---------------------------------------------------------------------------
 
+SFTP_CONNECT_TIMEOUT = 15  # seconds
+SFTP_BANNER_TIMEOUT = 15
+SFTP_AUTH_TIMEOUT = 30
+
+
 class _SftpAdapter(_FtpAdapter):
     def __init__(self, server: str, port: int, username: str, password: str):
-        self.transport = paramiko.Transport((server, port))
+        # Build socket with timeout to prevent hangs on unreachable hosts
+        import socket as _socket
+        sock = _socket.create_connection((server, port), timeout=SFTP_CONNECT_TIMEOUT)
+        self.transport = paramiko.Transport(sock)
+        self.transport.banner_timeout = SFTP_BANNER_TIMEOUT
+        self.transport.auth_timeout = SFTP_AUTH_TIMEOUT
         self.transport.connect(username=username, password=password)
         self.sftp = paramiko.SFTPClient.from_transport(self.transport)