feat(deploy): hourly off-site backups with restic

learn.halkyon.eu dumps its database every hour (people's files are in it
too) and restic stores the dump encrypted in a Hetzner Storage Box,
keeping 24 hourly, 7 daily, 4 weekly and 3 monthly snapshots. The box's
own snapshots, removable only in Hetzner's Console, protect the backups
from a taken-over server. A systemd timer runs it as deploy; once a day
it also prunes and checks the repository. Failures go to the journal.

Hourly, because a restore costs people who started after the backup
what their device held: the app no longer finds its session and starts
anew. deploy/backups.md describes the setup, the restore and what a
restore loses; both were rehearsed on the local Compose stack.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

README.md

@@ -73,7 +73,8 @@ licences require offering the source of what the instance runs to the people who
 
 learn.halkyon.eu itself deploys from `main` once CI is green: `deploy/redeploy.sh`, run by
 the CI runner described in `deploy/runner/`, deploys only commits signed by a key in
-`deploy/allowed-signers`.
+`deploy/allowed-signers`. It backs up its database every hour, off-site and encrypted;
+[`deploy/backups.md`](deploy/backups.md) says how to set that up and how to restore.
 
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