Deploy rearchitected MusicMouse backend + web frontend

The app grew a python-backend/ and a web/ frontend, so pi_musicmouse no
longer matches it. Rework the role:

- Install a pinned uv and build the venv with Python 3.13; Raspbian
  Bookworm's python3 is 3.11 and the backend uses PEP 695 syntax.
- pip install python-backend/ instead of a requirements.txt, and take
  the systemd unit straight from the checkout so it can't drift.
- Build web/ on the control machine (no npm on the Pi) and rsync
  web/dist over. Chain the checkout -> install -> build -> sync -> restart
  steps through handlers so a run with no repo change does nothing.
- Per-host config files (config-<host>.yml); the schema now differs
  between a host with a real mouse and a display-only one.
- Version to deploy is per host (musicmouse_version), no default.

Add pi_kiosk: autologin user running startx with Firefox in kiosk mode
and no window manager, for a Pi with a monitor attached. Enabled by
mediapi_has_monitor, defaulting off.

Turn musicdolphin into a display-only instance: serial and audio
simulated, driven entirely through the kiosk page.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

mediapis.yml

@@ -9,6 +9,8 @@
       when: mediapi_has_hifiberry_amp
     - role: pi_musicmouse
       when: mediapi_install_kidsmusic
+    - role: pi_kiosk
+      when: mediapi_has_monitor
     - pi_squeezelite_custom
     - pi_shairport
     - role: pi_irserver