docs: add authentication guide, TLS setup, FSTEC builds docs

- Fix docker-compose.yml: use ghcr.io/getnora-io/nora:latest
- Remove stale CHANGELOG.md.bak, add *.bak to .gitignore
- README: authentication guide (htpasswd, API tokens, RBAC)
- README: TLS/HTTPS section (reverse proxy, insecure-registries)
- README: document Dockerfile.astra and Dockerfile.redos (FSTEC)
- CHANGELOG: add 0.2.28 release notes

.gitignore

@@ -6,6 +6,9 @@ data/
 *.log
 internal config
 
+# Backup files
+*.bak
+
 # Internal files
 SESSION*.md
 TODO.md