Live-migrate a Proxmox VE VM to another cluster node via the API, checking preconditions first

Documented steps

1. Check migratability BEFORE acting: GET /nodes/{node}/qemu/{vmid}/migrate with target=<target node>. This is the precondition endpoint on the same path as the migration POST, and it reports blockers such as local resources and unavailable storages.
2. Confirm the target node is online: GET /cluster/status (entries have type 'node' or 'cluster'; node entries carry online and name).
3. Trigger the migration: POST /nodes/{node}/qemu/{vmid}/migrate with target=<node>.
4. For a running VM add online=1 to do a live migration with no downtime. The parameter is ignored if the VM is stopped (which is then a plain offline migration).
5. If the VM's disks are on local storage rather than shared storage, add with-local-disks=1 to stream the disks as part of the migration, and optionally targetstorage=<storage on target>.
6. Throttle with bwlimit (KiB/s) and choose migration_type=secure (SSH-tunnelled, default) or insecure (faster, only on a trusted isolated network).
7. The POST returns a UPID. Poll /nodes/{node}/tasks/{upid}/status until stopped, then verify exitstatus='OK' and that GET /cluster/resources?type=vm now shows the VM on the target node.