Replace stub provisioner with real Proxmox integration:
- qemu-img convert: source → QCOW2 in /mnt/converter/out/{vmid}/
- virt-resize --shrink --resize-force: auto-shrink >30 GiB disks
- qm create: VM with cores, RAM, network, SCSI controller
- qm importdisk: import each QCOW2 into Proxmox storage
- qm set: attach disks, configure boot (OVMF/SeaBIOS), EFI disk, serial
- Auto-destroy existing VM with same ID before recreating
- Background thread execution with in-memory status tracking
- Full logging throughout the conversion pipeline
