Document and ship a systemd unit file example for running memayu serve as a persistent service #49

Summary

Currently there's no guidance for keeping memayu serve running persistently and restarting it automatically (e.g. after a Raspberry Pi reboot). This doesn't need automation built into the binary itself — a documented, ready-to-copy systemd unit file is sufficient and is the standard expectation for this kind of self-hosted tool.

Fix

Add a contrib/systemd/memayu.service example file to the repository, and reference it from the README's self-hosted deployment section:

[Unit]
Description=Memayu memory server
After=network.target

[Service]
Type=simple
ExecStart=/usr/local/bin/memayu serve
Restart=on-failure
RestartSec=5
EnvironmentFile=/etc/memayu/.env
User=memayu

[Install]
WantedBy=multi-user.target

(Adjust paths/user to match actual installed conventions — this is illustrative, verify against how the binary is actually expected to be installed via Homebrew/npm/manual build.)

Document in the README: where to place this file, how to enable it (systemctl enable --now memayu), and how it interacts with .env (via EnvironmentFile, once Issue 1 in this batch is fixed and confirmed working) or config.toml.

Acceptance criteria

• contrib/systemd/memayu.service exists in the repo with working example content.
• README has a short "Running as a service" section linking to it, including enable/start/status/logs commands (systemctl status memayu, journalctl -u memayu -f).
• Manually verified on a real device (e.g. the Raspberry Pi used for this testing round) that the unit file works as documented: survives a reboot, restarts on crash.