Optional systemd unit for always-on recording

## Summary

Ship an optional **systemd** unit so machines that want continuous capture can run ltm under the service manager, without replacing ad-hoc `ltm start`.

Today: `ltm start` re-execs `daemon --foreground` with `Setsid` + pidfile. That stays the default for demos / one-off sessions.

## Proposal

- Add something like `deploy/ltm.service` (or `contrib/systemd/ltm.service`) that runs:
```
ExecStart=/usr/bin/ltm daemon --foreground --db … --pidfile …
```
- Document install/enable in `docs/cli.md` / `docs/recording.md` (Linux only; recording already requires Linux).
- Keep `ltm start` / `ltm stop` as the portable path; systemd is opt-in.
- Do **not** require systemd or add a Go daemonization library.

## Out of scope

- launchd / macOS (recording is Linux-only)
- Making systemd the only supported way to run the recorder

## Acceptance

- [ ] Unit file checked in with sane `User`/`CapabilityBoundingSet` notes (CAP_BPF / CAP_PERFMON or root)
- [ ] Docs: enable/start/stop via systemctl vs `ltm start`
- [ ] Confirmed `daemon --foreground` works as `Type=simple` ExecStart (already designed for this)