Add SSH connection validation for settings

backend/src/media_library_viewer_api/services/known_hosts.py

@@ -22,7 +22,7 @@ def _host_alias(host: str, port: int) -> str:
     return host if int(port or 22) == 22 else f"[{host}]:{int(port or 22)}"
 
 
-def _fetch_server_key(host: str, port: int, timeout: int = 10) -> paramiko.PKey:
+def _fetch_server_key(host: str, port: int, timeout: int = 30) -> paramiko.PKey:
     sock = socket.create_connection((host, int(port or 22)), timeout=timeout)
     transport = paramiko.Transport(sock)
     try:
@@ -31,6 +31,11 @@ def _fetch_server_key(host: str, port: int, timeout: int = 10) -> paramiko.PKey:
         if key is None:
             raise RuntimeError(f"Unable to read SSH host key for {host}:{port}")
         return key
+    except Exception as exc:
+        raise RuntimeError(
+            f"Unable to read SSH protocol banner from {host}:{port}. "
+            "Confirm the host is running an SSH server on that port and is reachable from the backend."
+        ) from exc
     finally:
         transport.close()
         sock.close()