licence,ci: ship OFL with Plex fonts and guard release dispatch (P5)

IBM Plex WOFF2 files were shipped without the SIL OFL 1.1 text, NOTICE
told modifiers to edit layout.html for a Source URL that lives in
legal.go, and workflow_dispatch took GITHUB_REF_NAME as the version so a
run from main would publish ghcr.io/...:main.

Closes P5 of docs/plans/code-review.md. CHANGELOG updated.

Co-Authored-By: Cursor Grok 4.6 <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

cmd/selfpost-backup/main.go

@@ -18,7 +18,6 @@
 //	docker exec -i <container> selfpost-backup -decrypt < backup.spbk > backup.tar.gz
 //
 // Copyright (C) 2026 Mikhail Yenuchenko
-// SPDX-License-Identifier: AGPL-3.0-only
 package main
 
 import (