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/pxe/ipxe.go

@@ -9,14 +9,15 @@ import (
 func BuildIPXEScript(publicURL string, img *model.Image, mac string) string {
 	kernelURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.KernelPath)
 	initrdURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.InitrdPath)
+	isoURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.ISOPath)
 
 	return fmt.Sprintf(`#!ipxe
 echo Provisioning: booting %s on ${mac}
 echo Loading kernel...
-kernel %s ramdisk_size=16777216 vga=791 video=vesafb:lfb:on rw quiet splash=verbose initrd=initrd
+kernel %s ramdisk_size=16777216 vga=791 video=vesafb:lfb:on rw quiet splash=verbose proxmox-iso-url=%s initrd=initrd
 echo Loading initrd...
 initrd --name initrd %s
 echo Booting...
 boot
-`, img.Name, kernelURL, initrdURL)
+`, img.Name, kernelURL, isoURL, initrdURL)
 }