feat(backup): add forgejo dump systemd timer + wire into runbook

Daily 'forgejo dump' (tar.zst) to /opt/backups/forgejo with 14-day retention,
via a oneshot service + timer. Maintenance section now documents both backup
layers (Codeberg push-mirror for git, dump timer for full state).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backup/forgejo-backup.sh

@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+# ----------------------------------------------------------------------------
+# forgejo-backup.sh — dump the live Forgejo instance and copy the archive to
+# the host. Run on the VPS via forgejo-backup.timer (see this dir).
+#
+# `forgejo dump` produces a single archive of the database, repositories,
+# config, and the data dir (attachments + the container registry blobs). It is
+# safe to run against a live instance.
+#
+# Install:
+#   sudo install -m 0755 forgejo-backup.sh /usr/local/bin/forgejo-backup.sh
+# ----------------------------------------------------------------------------
+set -euo pipefail
+
+CONTAINER=forgejo
+BACKUP_DIR=/opt/backups/forgejo
+RETAIN_DAYS=14
+STAMP="$(date +%Y%m%d-%H%M%S)"
+NAME="forgejo-${STAMP}.tar.zst"
+
+mkdir -p "$BACKUP_DIR"
+
+# Dump as the git user inside the container (writes to /tmp), then copy out.
+# Add --skip-package to exclude the registry blobs if dumps get too large
+# (they grow with per-commit :sha image tags — see registry GC in the README).
+docker exec -u git "$CONTAINER" \
+  forgejo dump --type tar.zst --skip-log --file "/tmp/${NAME}"
+docker cp "${CONTAINER}:/tmp/${NAME}" "${BACKUP_DIR}/${NAME}"
+docker exec "$CONTAINER" rm -f "/tmp/${NAME}"
+
+# Retention: drop dumps older than RETAIN_DAYS.
+find "$BACKUP_DIR" -maxdepth 1 -name 'forgejo-*.tar.zst' -mtime +"${RETAIN_DAYS}" -delete
+
+echo "Backup written: ${BACKUP_DIR}/${NAME}"