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>

README.md

@@ -13,12 +13,14 @@ sudo terractl attach calamity
 
 - **Many servers, one host.** Each instance is `terraria@<name>.service`, with its own directory, port, world and mods.
 - **Vanilla and tModLoader side by side**, chosen per instance.
-- **No manual installs.** Drop the official server zips into `/var/lib/terraria/dist`; they're unpacked on demand, and each instance can be pinned to a specific version. `terractl upgrade` checks a new zip, backs up the worlds and switches version in one step.
+- **No manual installs.** `terractl fetch` downloads official Terraria and tModLoader server zips (or drop them into `/var/lib/terraria/dist` yourself); they're unpacked on demand, and each instance can be pinned to a specific version. `terractl upgrade` checks a new zip, backs up the worlds and switches version in one step.
 - **Safe shutdowns.** Stopping a server saves the world first.
 - **A live console.** `terractl attach` shows the server's output and lets you type commands into it; `terractl watch` just shows the output.
 - **Mod management.** `terractl mods` installs, enables and removes mods, with correct file ownership.
 - **Tab completion** for commands, instance names, mods and console commands.
 - **Export and import.** Pack an instance into one `.tar.gz` for backups or moving to another machine, and import it anywhere, with paths, ports and ownership adjusted.
+- **Scheduled backups.** `terractl backup enable <name>` backs an instance up daily (or on any schedule) with a systemd timer, saving the world first and keeping the newest 14.
+- **Self-check.** `terractl doctor` checks the whole setup, explains what's wrong with the command to fix it, and `--fix` fixes what it safely can.
 - **Manage other hosts.** Run any command on another host with `terractl --host nas ...`, and copy or move instances between hosts with `terractl copy` and `terractl move`, like LXD/Incus remotes. Hosts opt in by running the `terractld` daemon; access is per computer, over pinned TLS.
 - **Hardened.** Servers run as an unprivileged `terraria` user and can only write to their own data.
 - **Access for moderators.** Members of the `terraria` group can start, stop and restart servers, use the console and read the servers' own logs, without `sudo` and without seeing the rest of the system's logs.