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>

lib/terractl-remote

@@ -325,7 +325,7 @@ def host_addresses(listen_host, port):
 # --------------------------------------------------------------- daemon ----
 
 EXEC_COMMANDS = {"list", "info", "new", "start", "stop", "restart", "enable", "disable", "cmd", "watch",
-                 "mods", "reinstall", "upgrade", "fix-perms", "delete", "rename", "export", "import",
+                 "mods", "reinstall", "upgrade", "fix-perms", "doctor", "backup", "fetch", "delete", "rename", "export", "import",
                  "-V", "--version", "version", "-h", "--help", "help"}