deploy: version systemd runtime setup

deploy/systemd/README.md

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+# systemd deployment
+
+This directory contains the versioned systemd deployment used for the Debian runtime.
+
+## Install
+
+Run the installer as `root` and point it at the live runtime root:
+
+```bash
+cd /path/to/m2dev-server
+python3 deploy/systemd/install_systemd.py \
+  --user mt2.jakubkadlec.dev \
+  --group mt2.jakubkadlec.dev \
+  --runtime-root /home/mt2.jakubkadlec.dev/metin/runtime/server \
+  --channel 1 \
+  --channel 99 \
+  --restart
+```
+
+`--channel-limit 1` is also supported and will auto-include channel `99` when present in `channels.py`.
+
+## What it installs
+
+- `metin-server.service`
+- `metin-db.service`
+- `metin-db-ready.service`
+- `metin-auth.service`
+- `metin-game@.service`
+- `/usr/local/libexec/metin-game-instance-start`
+- `/usr/local/libexec/metin-wait-port`
+
+The `metin-db-ready.service` gate waits until the DB socket is actually accepting connections before `auth` and `game` units start.