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>

CHANGELOG.md

@@ -1,10 +1,23 @@
 # Changelog
 
-## 0.0.12 - unreleased
+## 0.0.12 - 2026-10-02
 
+- New `fetch` command to download server zips into `dist/`, named with their version:
+  - `terractl fetch tmodloader [<version>]` downloads from tModLoader's GitHub releases (default: the latest stable) and checks it against the SHA-256 checksum GitHub publishes. `terractl fetch vanilla [<version>]` downloads from terraria.org (default: its newest; older versions by number, `1449` or `1.4.4.9`).
+  - `--list` shows the versions available (tModLoader: stable and preview), and which are already downloaded. `--upgrade <name>` switches an instance to the zip with `upgrade` right after.
+  - Every zip is checked (complete, and the right kind) before it's put in `dist/`; zips already there aren't downloaded again. If a site doesn't have the version or can't be reached, it says so and where to download it by hand.
+  - Members of the `terraria` group can use it without `sudo` (except `--upgrade`). Needs `curl`.
+- Scheduled backups, with a systemd timer per instance (`terraria-backup@<name>.timer`):
+  - `sudo terractl backup enable <name> [--every <when>] [--keep <number>]` (default: daily, keeping the newest 14). `--every` takes `hourly`, `daily`, `weekly`, a time such as `04:30`, or any systemd calendar expression. Missed backups are made after the machine starts again.
+  - `backup now <name>` makes one immediately, `backup disable <name>` stops the schedule (existing backups are kept), and `backup list [<name>]` shows schedules, the last and next run, and the backups.
+  - Each backup saves the world first if the server is running (as `export` does) and goes in `/var/lib/terraria/backups/<name>/` (`TERRARIA_BACKUPS` in `/etc/default/terractl`), readable by the `terraria` group. Only scheduled backups are pruned.
+  - `rename` takes the schedule along, `delete` turns it off, and purging the package removes the schedules (not the backups).
+- New `doctor` command: `terractl doctor [<name>] [--fix]` checks the system (user, programs including ICU and curl, folders, zips, the systemd units and old copies in `/etc` that replace them, sandbox access to moved folders, journal access, polkit, the daemon, leftovers of deleted instances) and each instance (file ownership and modes, its server zip, paths in `serverconfig.txt`, missing mods, shared ports, failed starts and backups). Each problem comes with the command that fixes it. `--fix` applies the safe fixes: `fix-perms`, folder and zip permissions, journal access, the `ReadWritePaths=` drop-in for moved folders, and boot links or backup schedules of instances that are gone. It exits with status 1 if a problem remains.
+- The daemon (`terractld.service`) may now write to `/etc/systemd/system`, so `backup enable` and `doctor --fix` also work through `--host`.
+- Warnings about moved folders now also suggest `terractl doctor --fix`.
 - New `upgrade` command to change an instance's server version, up or down:
   - `terractl upgrade <name>` lists the zips in `dist/` for its type, marking the installed, pinned and newest ones. The `terraria` group can use it without `sudo`.
-  - `sudo terractl upgrade <name> <zip>|latest` checks the zip first (right type, readable by `terraria`, contains the server, passes `unzip -t`), stops the server (saving the world), backs up its worlds and configs to `instances/.backups/`, pins `ZIP=`, unpacks the zip right away and starts the server again if it was running. If unpacking fails, the instance is left as it was. `--no-backup` skips the backup.
+  - `sudo terractl upgrade <name> <zip>|latest` checks the zip first (right type, readable by `terraria`, contains the server, passes `unzip -t`), stops the server (saving the world), backs up its worlds and configs to `/var/lib/terraria/backups/<name>/`, pins `ZIP=`, unpacks the zip right away and starts the server again if it was running. If unpacking fails, the instance is left as it was. `--no-backup` skips the backup.
   - Going to an older version needs `--downgrade`, because worlds saved by a newer version can't be opened by an older one.
   - Bash completion offers the instance's zips. Works with `--host`.
 - `reinstall` now always unpacks the **same** version again. Before, an instance with a blank `ZIP=` picked the newest zip in `dist/`, so `reinstall` could silently upgrade it. It now pins the zip the instance runs first, and refuses if that zip is gone from `dist/`. It also keeps the record of the installed version (`server/.installed`) until the new unpack is done.