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/httpserver/router.go

@@ -73,6 +73,7 @@ func NewRouter(d Deps) http.Handler {
 
 	// Boot / PXE endpoints
 	r.Get("/ipxe/{mac}", d.BootAPI.IPXEScript)
+	r.Get("/api/boot/auto-answer", d.BootAPI.AutoAnswer)
 	r.Post("/api/boot/answer", d.BootAPI.AnswerFile)
 	r.Post("/api/hosts/{id}/installed", d.BootAPI.InstallComplete)
 	r.Get("/api/hosts/{id}/first-boot-script", d.BootAPI.FirstBootScript)