add doctor, scheduled backups and fetch; release 0.0.12

- doctor [<name>] [--fix]: checks the system (user, programs, folders,
  zips, units and leftover copies in /etc, sandbox access to moved
  folders, journal, polkit, daemon, orphaned boot links and schedules)
  and each instance (ownership, server zip, config paths, mods, ports,
  failed starts/backups), with a fix for each. --fix applies the safe
  ones and re-checks them.
- backup enable|disable|now|list: a systemd timer per instance
  (terraria-backup@.timer, Persistent=), saving the world first, into
  /var/lib/terraria/backups/<name>/ (group-readable), pruning only
  scheduled backups. rename carries the schedule, delete removes it.
  upgrade's backups move there too.
- fetch vanilla|tmodloader [<version>] [--list] [--upgrade <name>]:
  downloads from terraria.org / tModLoader's GitHub releases, checks
  GitHub's sha256 and the zip, names it with its version.
- terractld may write /etc/systemd/system, for backup enable and
  doctor --fix over --host.
- Units, tmpfiles, packaging, completion, docs and changelog.

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

packaging/deb/postrm

@@ -6,6 +6,9 @@ case "$1" in
         # Forget which instances were enabled at boot
         rm -f /etc/systemd/system/multi-user.target.wants/terraria@*.service
         rm -rf /etc/systemd/system/terraria@.service.d
+        # Scheduled backups (the backups themselves stay in /var/lib/terraria/backups)
+        rm -f /etc/systemd/system/timers.target.wants/terraria-backup@*.timer
+        rm -rf /etc/systemd/system/terraria-backup@*.timer.d /etc/systemd/system/terraria-backup@*.service.d
         # The daemon: whether it's on, its certificate and the computers it trusts
         rm -f /etc/systemd/system/multi-user.target.wants/terractld.service
         rm -rf /etc/systemd/system/terractld.service.d /var/lib/terractl

packaging/deb/prerm

@@ -8,6 +8,7 @@ case "$1" in
     remove|deconfigure)
         if [ -d /run/systemd/system ]; then
             systemctl stop terractld.service || true
+            systemctl stop 'terraria-backup@*.timer' 'terraria-backup@*.service' || true
             units=$(systemctl list-units --plain --no-legend --all 'terraria@*.service' | awk '{print $1}')
             if [ -n "$units" ]; then
                 echo "Stopping Terraria instances: $units"