Add systemd service files and Raspberry Pi installation guide

[Gudsfile]

Problem

No official path to run jukebox as a system service. Users must hand-craft their own .service files and figure out uv path resolution themselves. This is the main friction point for Raspberry Pi deployments.

Proposed change

1. Add jukebox.service and jukebox-admin.service template files to the repo (under deploy/systemd/ or similar), with uv run correctly wired and Environment= for config.
2. Add an install script (install.sh) that copies the service files, runs systemctl enable, and substitutes the correct paths. No Python CLI involvement.
3. Document the full Raspberry Pi setup flow in the README or a dedicated docs/raspberry-pi.md.

Out of scope

• jukebox start/stop CLI wrappers (redundant with systemctl)
• The app managing its own lifecycle