feat(backup): add rsync --link-dest off-host transport, restore docs

backup.sh now supports BACKUP_REMOTE_RSYNC alongside BACKUP_REMOTE.
The rsync path writes <root>/YYYY-MM-DD/<archive> on the target with
--link-dest pointing at the previous day's directory, so unchanged
archives become hard links and daily snapshots cost almost zero
extra bytes. BACKUP_REMOTE_SSH_KEY routes the rsync ssh leg to a
dedicated identity (e.g. rsync.net restricted accounts).

Timer moved to 03:00 Europe/Berlin (was 03:17 UTC) per #41.

docs/operations.md: full restore procedure (parallel stack first,
then atomic swap) plus the rsync vs rclone trade-off. Closes most
of #41 — the only remaining task is the operator's choice of
off-host target.

scripts/backup.sh

@@ -13,10 +13,18 @@
 # Optional env:
 #   BACKUP_DIR           where to write archives (default /var/backups/librenotes)
 #   BACKUP_REMOTE        rclone target for off-site copy (e.g. s3:bucket/path)
+#   BACKUP_REMOTE_RSYNC  rsync destination for hard-link-deduplicated
+#                        off-host copy, e.g. backup@rsync.net:librenotes/
+#                        Layout written at the target:
+#                          <root>/YYYY-MM-DD/librenotes-<ts>.tar.gz
+#                        Each run passes --link-dest=../<previous>/ so
+#                        unchanged archives cost zero extra bytes.
+#   BACKUP_REMOTE_SSH_KEY path to a private SSH key used for the rsync
+#                        leg (passed via -e "ssh -i <key>").
 #   BACKUP_VERSION       version string written into info.txt
 #
 # The script is intentionally a single self-contained file so it
-# can run on a minimal host with only sqlite3, tar, gzip, and
+# can run on a minimal host with only sqlite3, tar, gzip, rsync, and
 # (optionally) rclone.
 
 set -euo pipefail
@@ -58,3 +66,22 @@ if [ -n "${BACKUP_REMOTE:-}" ]; then
   rclone copy "$archive" "$BACKUP_REMOTE" --quiet
   echo "uploaded to $BACKUP_REMOTE"
 fi
+
+if [ -n "${BACKUP_REMOTE_RSYNC:-}" ]; then
+  # Compute today / previous-day directory names. Layout at the
+  # remote target is <root>/YYYY-MM-DD/. We pass --link-dest pointing
+  # at yesterday's dir so unchanged archives become hard links —
+  # ~free for daily snapshots that are mostly identical.
+  today="$(date -u +%Y-%m-%d)"
+  yesterday="$(date -u -d 'yesterday' +%Y-%m-%d 2>/dev/null \
+              || date -u -v-1d +%Y-%m-%d)"
+  ssh_opts=()
+  if [ -n "${BACKUP_REMOTE_SSH_KEY:-}" ]; then
+    ssh_opts=(-e "ssh -i $BACKUP_REMOTE_SSH_KEY -o StrictHostKeyChecking=accept-new")
+  fi
+  # Strip any trailing slash so we control the join.
+  remote="${BACKUP_REMOTE_RSYNC%/}"
+  rsync -a --link-dest="../$yesterday/" "${ssh_opts[@]}" \
+        "$archive" "$remote/$today/"
+  echo "rsync'd $archive -> $remote/$today/"
+fi