Add automated PXE installation via ISO GRUB modification and auto-answer endpoint

Modifies uploaded ISO's GRUB config in-place to set timeout=0 and inject
proxmox-start-auto-installer + answer-url kernel params, enabling fully
hands-off installation. Adds /api/boot/auto-answer endpoint that identifies
hosts by ARP-resolving the requester's IP to MAC address.

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

internal/image/isomod.go

@@ -0,0 +1,169 @@
+package image
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"strings"
+
+	"github.com/kdomanski/iso9660"
+)
+
+func ModifyISOForAutoInstall(isoPath, answerURL string) error {
+	origConfig, err := readGrubCfgFromISO(isoPath)
+	if err != nil {
+		return err
+	}
+	if origConfig == nil {
+		log.Printf("image: no grub.cfg found in ISO, skipping auto-install modification")
+		return nil
+	}
+
+	newConfig := rewriteGrubConfig(string(origConfig), answerURL)
+
+	if len(newConfig) > len(origConfig) {
+		return fmt.Errorf("modified GRUB config (%d bytes) exceeds original (%d bytes)", len(newConfig), len(origConfig))
+	}
+
+	padded := make([]byte, len(origConfig))
+	copy(padded, []byte(newConfig))
+	for i := len(newConfig); i < len(padded); i++ {
+		padded[i] = '\n'
+	}
+
+	offset, err := findContentInISO(isoPath, origConfig)
+	if err != nil {
+		return fmt.Errorf("locate grub.cfg in ISO: %w", err)
+	}
+
+	f, err := os.OpenFile(isoPath, os.O_WRONLY, 0)
+	if err != nil {
+		return fmt.Errorf("open ISO for writing: %w", err)
+	}
+	defer f.Close()
+
+	if _, err := f.WriteAt(padded, offset); err != nil {
+		return fmt.Errorf("write modified grub.cfg: %w", err)
+	}
+
+	log.Printf("image: modified grub.cfg at offset %d for auto-install", offset)
+	return nil
+}
+
+func readGrubCfgFromISO(isoPath string) ([]byte, error) {
+	f, err := os.Open(isoPath)
+	if err != nil {
+		return nil, fmt.Errorf("open ISO: %w", err)
+	}
+	defer f.Close()
+
+	img, err := iso9660.OpenImage(f)
+	if err != nil {
+		return nil, fmt.Errorf("parse ISO: %w", err)
+	}
+
+	root, err := img.RootDir()
+	if err != nil {
+		return nil, fmt.Errorf("read ISO root: %w", err)
+	}
+
+	grubFile := findFileByName(root, "grub.cfg")
+	if grubFile == nil {
+		return nil, nil
+	}
+
+	reader := grubFile.Reader()
+	data, err := io.ReadAll(reader)
+	if err != nil {
+		return nil, fmt.Errorf("read grub.cfg: %w", err)
+	}
+	return data, nil
+}
+
+func findFileByName(dir *iso9660.File, target string) *iso9660.File {
+	children, err := dir.GetChildren()
+	if err != nil {
+		return nil
+	}
+	for _, child := range children {
+		if child.IsDir() {
+			if result := findFileByName(child, target); result != nil {
+				return result
+			}
+		} else if strings.EqualFold(child.Name(), target) {
+			return child
+		}
+	}
+	return nil
+}
+
+func rewriteGrubConfig(original, answerURL string) string {
+	lines := strings.Split(original, "\n")
+	var result []string
+	depth := 0
+
+	for _, line := range lines {
+		trimmed := strings.TrimSpace(line)
+
+		if strings.HasPrefix(trimmed, "set timeout=") {
+			result = append(result, "set timeout=0")
+			continue
+		}
+		if strings.HasPrefix(trimmed, "set default=") {
+			result = append(result, "set default=0")
+			continue
+		}
+
+		if strings.HasPrefix(trimmed, "menuentry") {
+			depth++
+		}
+		if trimmed == "}" && depth > 0 {
+			depth--
+		}
+
+		if depth == 1 && strings.Contains(trimmed, "linux ") && strings.Contains(trimmed, "/boot/linux") {
+			if !strings.Contains(line, "proxmox-start-auto-installer") {
+				line = strings.TrimRight(line, " \t") + " proxmox-start-auto-installer"
+			}
+			if answerURL != "" && !strings.Contains(line, "proxmox-auto-installer-answer-url") {
+				line += " proxmox-auto-installer-answer-url=" + answerURL
+			}
+		}
+
+		result = append(result, line)
+	}
+
+	return strings.Join(result, "\n")
+}
+
+func findContentInISO(isoPath string, content []byte) (int64, error) {
+	f, err := os.Open(isoPath)
+	if err != nil {
+		return 0, err
+	}
+	defer f.Close()
+
+	prefix := content
+	if len(prefix) > 256 {
+		prefix = prefix[:256]
+	}
+
+	buf := make([]byte, 2048)
+	stat, _ := f.Stat()
+
+	for offset := int64(0); offset < stat.Size(); offset += 2048 {
+		n, err := f.ReadAt(buf, offset)
+		if n == 0 {
+			break
+		}
+		if err != nil && err != io.EOF {
+			return 0, err
+		}
+		if bytes.HasPrefix(buf[:n], prefix) {
+			return offset, nil
+		}
+	}
+	return 0, fmt.Errorf("grub.cfg content not found in ISO raw data")
+}