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/workers/monitoring_worker.py

@@ -737,7 +737,10 @@ class MonitoringWorker:
                         existing.os_type = os_type
                         existing.vm_type = vm_type
                         existing.is_template = bool(vm_data.get('template'))
-                        existing.ip_address = ip_address
+                        # Only overwrite ip_address if IPAM has a value;
+                        # otherwise preserve user-assigned IP from deploy/wizard
+                        if ip_address:
+                            existing.ip_address = ip_address
                         existing.last_sync_at = sync_time
                         existing.deleted_at = None
                     else: