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>

cmd/provisioning/main.go

@@ -56,7 +56,7 @@ func main() {
 	images := &store.Images{DB: database}
 	activity := &store.Activity{DB: database}
 
-	imageSvc := &image.Service{Store: images, ImageDir: cfg.Images.Dir}
+	imageSvc := &image.Service{Store: images, ImageDir: cfg.Images.Dir, PublicURL: cfg.Server.PublicURL}
 
 	hub := events.NewHub()