Run the media backup as uid 10001 and add the deploy runbook

Address review of 9ac3342:

- The media backup now runs as the host user mealquest (10001) with no
  capabilities. systemd bind-mounts the data volume read-only at
  /run/mealquest-data and passes the NAS key as a credential; the script
  changes into media, refuses it if the physical path differs (symlink)
  and rsyncs "./", so the path cannot be swapped under it. ssh pins the
  NAS host keys from /etc/mealquest/known_hosts (deploy/known_hosts).
- A missing media directory now fails the job. The deploy step creates it.
- deploy/RUNBOOK.md records the Traefik route (task 0c1dc172), backup
  setup, the restore procedure (stop, move db/-wal/-shm aside, restore,
  count check, start) and the 2026-10-05 drill with ltx -level all.

Task: d2f2e87e-5114-4ef1-93ab-360ffb5c009e

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/systemd/mealquest-media-backup.service

@@ -3,6 +3,11 @@
 #   systemctl start mealquest-media-backup
 # and read the result with
 #   journalctl -u mealquest-media-backup
+#
+# The script runs as the app's user (10001, host user "mealquest") with no
+# capabilities. systemd, as root, only bind-mounts the data volume's root
+# (a root-owned path the app cannot replace) read-only at
+# /run/mealquest-data and copies the NAS key into the credentials directory.
 [Unit]
 Description=Back up MealQuest media to the Synology NAS
 After=network-online.target
@@ -11,3 +16,14 @@ Wants=network-online.target
 [Service]
 Type=oneshot
 ExecStart=/usr/local/sbin/mealquest-media-backup
+User=mealquest
+Group=mealquest
+LoadCredential=nas_key:/home/overseer/mealquest/deploy/secrets/litestream_key
+BindReadOnlyPaths=/var/lib/docker/volumes/mealquest-data/_data:/run/mealquest-data
+ProtectSystem=strict
+ProtectHome=yes
+PrivateTmp=yes
+PrivateDevices=yes
+NoNewPrivileges=yes
+CapabilityBoundingSet=
+RestrictAddressFamilies=AF_INET AF_INET6 AF_UNIX