Add boot image management with ISO extraction and serving

Upload Proxmox ISOs via API or dashboard UI, extract kernel+initrd
using pure-Go iso9660 library, store on disk, and serve over HTTP
for PXE booting. Dynamic kernel/initrd filenames per image replace
the previous hardcoded paths.

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

internal/pxe/ipxe.go

@@ -7,8 +7,8 @@ import (
 )
 
 func BuildIPXEScript(publicURL string, img *model.Image, mac string) string {
-	kernelURL := fmt.Sprintf("%s/images/boot/%s/%s", publicURL, img.Name, "linux26")
-	initrdURL := fmt.Sprintf("%s/images/boot/%s/%s", publicURL, img.Name, "initrd.img")
+	kernelURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.KernelPath)
+	initrdURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.InitrdPath)
 
 	return fmt.Sprintf(`#!ipxe
 echo Provisioning: booting %s on ${mac}