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/image/extract.go

@@ -13,6 +13,7 @@ import (
 type ExtractResult struct {
 	KernelFilename string
 	InitrdFilename string
+	ISOFilename    string
 }
 
 type ProgressFunc func(stage string, detail string)
@@ -33,22 +34,21 @@ func ExtractFromISOWithProgress(r io.Reader, destDir string, progress ProgressFu
 
 	report("receiving", "Writing ISO to disk...")
 
-	tmp, err := os.CreateTemp(filepath.Dir(destDir), "iso-upload-*.tmp")
+	isoPath := filepath.Join(destDir, "original.iso")
+	tmp, err := os.Create(isoPath)
 	if err != nil {
-		return nil, fmt.Errorf("create temp file: %w", err)
+		return nil, fmt.Errorf("create ISO file: %w", err)
 	}
-	tmpPath := tmp.Name()
-	defer os.Remove(tmpPath)
 
 	if _, err := io.Copy(tmp, r); err != nil {
 		tmp.Close()
-		return nil, fmt.Errorf("write ISO to temp file: %w", err)
+		return nil, fmt.Errorf("write ISO to disk: %w", err)
 	}
 	tmp.Close()
 
 	report("parsing", "Parsing ISO image...")
 
-	f, err := os.Open(tmpPath)
+	f, err := os.Open(isoPath)
 	if err != nil {
 		return nil, fmt.Errorf("open temp ISO: %w", err)
 	}
@@ -102,6 +102,7 @@ func ExtractFromISOWithProgress(r io.Reader, destDir string, progress ProgressFu
 	return &ExtractResult{
 		KernelFilename: kernelName,
 		InitrdFilename: initrdName,
+		ISOFilename:    "original.iso",
 	}, nil
 }