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/api/boot.go

@@ -1,11 +1,14 @@
 package api
 
 import (
+	"bufio"
 	"encoding/json"
 	"errors"
 	"fmt"
 	"log"
+	"net"
 	"net/http"
+	"os"
 	"strings"
 
 	"provisioning/internal/config"
@@ -169,6 +172,68 @@ func (a *BootAPI) PhoneHome(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, map[string]any{"ok": true})
 }
 
+func (a *BootAPI) AutoAnswer(w http.ResponseWriter, r *http.Request) {
+	clientIP, _, _ := net.SplitHostPort(r.RemoteAddr)
+	if clientIP == "" {
+		clientIP = r.RemoteAddr
+	}
+
+	mac := macFromARP(clientIP)
+	if mac == "" {
+		log.Printf("auto-answer: no ARP entry for %s", clientIP)
+		http.Error(w, "could not determine MAC for your IP", http.StatusNotFound)
+		return
+	}
+
+	host, err := a.Hosts.GetByMAC(r.Context(), mac)
+	if err != nil {
+		log.Printf("auto-answer: MAC %s (IP %s) not registered", mac, clientIP)
+		http.Error(w, "unknown host", http.StatusNotFound)
+		return
+	}
+
+	st, ok := a.ServerTypes.Get(host.ServerType)
+	if !ok {
+		http.Error(w, "unknown server type", http.StatusInternalServerError)
+		return
+	}
+
+	if host.State == model.StatePXEBooted || host.State == model.StatePXEReady {
+		a.Runner.Transition(r.Context(), host.ID, statemachine.TriggerPXEScriptServed)
+		a.Runner.Transition(r.Context(), host.ID, statemachine.TriggerAnswerServed)
+		a.Runner.LogActivity(r.Context(), host.ID, model.LogInfo, "pxe", "Auto-installer answer file served")
+	}
+
+	_, pubKey, _ := a.Hosts.GetEphemeralKey(r.Context(), host.ID)
+	if pubKey == "" {
+		http.Error(w, "no ephemeral key for host", http.StatusInternalServerError)
+		return
+	}
+
+	log.Printf("auto-answer: serving answer for %s (%s) from IP %s", host.Hostname, mac, clientIP)
+	answer := pxe.GenerateAnswerFile(host, st, a.Config, pubKey)
+	w.Header().Set("Content-Type", "application/toml")
+	w.Write([]byte(answer))
+}
+
+func macFromARP(ip string) string {
+	f, err := os.Open("/proc/net/arp")
+	if err != nil {
+		return ""
+	}
+	defer f.Close()
+
+	scanner := bufio.NewScanner(f)
+	scanner.Scan() // skip header
+	for scanner.Scan() {
+		fields := strings.Fields(scanner.Text())
+		if len(fields) >= 4 && fields[0] == ip && fields[3] != "00:00:00:00:00:00" {
+			return strings.ToLower(fields[3])
+		}
+	}
+	return ""
+}
+
 func normalizeMAC(m string) string {
 	m = strings.ToLower(strings.TrimSpace(m))
 	m = strings.ReplaceAll(m, "-", ":")