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/db/migrations/0004_image_iso_path.sql

@@ -0,0 +1 @@
+ALTER TABLE images ADD COLUMN iso_path TEXT NOT NULL DEFAULT '';