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>

roles/pi_musicmouse/defaults/main.yml

@@ -0,0 +1,15 @@
+---
+pi_musicmouse_repo: "ssh://git@git.bauer.tech:2222/martin/musicmouse.git"
+# No default on purpose: which tag/branch to run is a per-host decision, set in host_vars.
+# musicmouse_version: "rearchitect-backend"
+
+pi_musicmouse_python_version: "3.13"
+pi_musicmouse_uv_version: "0.12.10"
+
+# Where to check out the repo on the *control machine* to build the frontend (npm isn't
+# installed on the Pi - see README). Keyed by version so different hosts on different
+# versions don't clobber each other's build.
+pi_musicmouse_build_cache: "/tmp/musicmouse-build-cache"
+
+# Which files/config-*.yml to install as this host's /media/musicmouse/config.yml.
+pi_musicmouse_config_file: "config-{{ inventory_hostname }}.yml"