fix(lxc): preserve credentials and IP across reinstall

Three related fixes so login/password/SSH key/IP survive a reinstall:

1. backend/app/api/proxmox/lxc.py — on initial LXC deploy save
   cloud_init_user='root' and cloud_init_password=req.password to VMInstance
   (previously both were None, so reinstall could not restore root password).

2. backend/app/api/proxmox/async_ops.py — _do_reinstall_sync passes
   cached.cloud_init_password to create_lxc_container so the new container
   gets the same root password as the original.

3. backend/app/workers/monitoring_worker.py — VM sync worker no longer
   overwrites VMInstance.ip_address with NULL when no IPAM allocation
   exists for that (server_id, vmid). It only updates the field when
   IPAM has a value, otherwise the IP saved at deploy/wizard time is
   preserved.

backend/app/api/proxmox/async_ops.py

@@ -221,6 +221,7 @@ def _do_reinstall_sync(task_id: int, server_id: int, vmid: int, node: str,
                     net0=net0,
                     nameserver=nameserver,
                     searchdomain=searchdomain,
+                    password=cached.cloud_init_password,
                     ssh_public_keys=cached.ssh_keys,
                     unprivileged=unprivileged,
                     start_after_create=False,