Serve full ISO for PXE boot via proxmox-iso-url kernel param

The Proxmox installer needs the full ISO to access packages and
installer data. Previously the ISO was deleted after extracting
kernel+initrd. Now we keep it as original.iso and serve it via HTTP.
The iPXE script passes proxmox-iso-url=<url> so the installer
fetches the ISO over the network instead of scanning block devices.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/model/model.go

@@ -62,6 +62,7 @@ type Image struct {
 	Version    string
 	KernelPath string
 	InitrdPath string
+	ISOPath    string
 	IsDefault  bool
 	CreatedAt  time.Time
 }