chore(release): archive ISO build recipes, tarball-only releases

Releases no longer ship as bootable ISOs. Archipelago updates are
distributed as the backend binary plus a frontend tarball referenced by
releases/manifest.json. Nodes OTA-update via scripts/self-update.sh.

Filebrowser and AIUI remain bundled inside the frontend tarball and
deployed atomically, verified present in v1.7.43-alpha release artifact
(189 AIUI files, filebrowser-client bundle).

Archived under image-recipe/_archived/ (resurrectable if ISO distribution
is reintroduced):
  - build-auto-installer-iso.sh
  - build-unbundled-iso.sh
  - test-iso-qemu.sh
  - scripts/convert-iso-to-disk.sh
  - BUILD-ISO-STATUS.md, ISO-BUILD-CHECKLIST.md
  - branding/isohdpfx.bin
  - .gitea/workflows/build-iso-dev.yml

Updated release process docs to drop ISO references:
  - scripts/create-release.sh (next-steps text)
  - docs/BETA-RELEASE-CHECKLIST.md
  - docs/hotfix-process.md
  - README.md

docs/BETA-RELEASE-CHECKLIST.md

@@ -20,7 +20,6 @@
 - [ ] `core/security/src/secrets_manager.rs` — encryption + rotation
 - [ ] `neode-ui/src/views/Marketplace.vue` — all app entries with pinned image versions
 - [ ] `neode-ui/src/api/websocket.ts` — heartbeat + reconnection
-- [ ] `image-recipe/build-auto-installer-iso.sh` — all container images captured
 - [ ] `image-recipe/configs/nginx-archipelago.conf` — all app proxies + path traversal blocks
 - [ ] All app icons present in `neode-ui/public/assets/img/app-icons/`