fix: add SFTP timeouts + prevent /api/pdf hang

backend/sftp_store.py

@@ -1,4 +1,5 @@
 import os
+import socket
 import posixpath
 import paramiko
 
@@ -23,7 +24,9 @@ def download_bytes(remote_path: str) -> bytes:
 
     rp = remote_path.lstrip("/")
 
-    transport = paramiko.Transport((host, port))
+    transport = paramiko.Transport(socket.create_connection((host, port), timeout=10))
+    transport.banner_timeout = 10
+    transport.auth_timeout = 10
     transport.connect(username=user, password=pw)
     sftp = paramiko.SFTPClient.from_transport(transport)