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>

config/default/terractl

@@ -13,6 +13,9 @@
 # Where server zips are read from. Outside /home, no drop-in is needed.
 #TERRARIA_DIST=/var/lib/terraria/dist
 
+# Where 'terractl backup' (and 'upgrade', before switching) puts backups
+#TERRARIA_BACKUPS=/var/lib/terraria/backups
+
 # Seconds to wait for a server to save and exit on stop (keep below 90)
 #TERRARIA_STOP_TIMEOUT=80