Move the 2011 rdiff-backup archival post from _hakyll/ to _nikola/

## Summary

`src/data/blog/_hakyll/using-rdiff-backup-backup-remote-clients-with-ease.md` is a 2011 Nikola-era post filed under `_hakyll/`, and its stanza reads "From this blog's Hakyll era". It belongs in `_nikola/` alongside its siblings.

## Motivation

The restore in #167 predated any `_nikola/` directory, so `_hakyll/` was the only archival home available. #316 then restored three posts of the same vintage — `using-apc-to-speed-up-php`, `using-memcached-with-mediawiki-and-wordpress`, and `best-practices-bind-mounts-and-chroots`, all written within three days of the rdiff-backup post and moved out of `nikola/posts/` by the same 2017 commit (27dfa43) — into `_nikola/`. One post now claims the wrong era.

`docs/reference/post-frontmatter.md` defines the directory as the engine of origin, and the source for all four is `nikola/posts/*.rst`.

## Scope

- Move the file to `src/data/blog/_nikola/`.
- Change the stanza's engine name from Hakyll to Nikola.
- No URL change: `_`-prefixed directories are stripped, so the post keeps serving at `/posts/using-rdiff-backup-backup-remote-clients-with-ease/`.

Out of scope: `_hakyll/how-i-read.md` is a 2018 post and genuinely Hakyll-era. After this move `_hakyll/` holds that one post.

## Acceptance criteria

- [ ] The post lives under `src/data/blog/_nikola/`
- [ ] Its stanza names the Nikola era
- [ ] `/posts/using-rdiff-backup-backup-remote-clients-with-ease/` still returns 200