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/README.md

@@ -1,4 +1,20 @@
 # pi_musicmouse
 
-Deploys the "Musicmouse" application: checks out its git repo, creates a
-Python virtualenv, sets up a media directory, and installs its config file.
+Deploys the MusicMouse backend (`python-backend/`) and its built web frontend.
+
+- Checks out `musicmouse_version` (a tag or branch - set per host in `host_vars`, no
+  default) from `musicmouse_repo` to `/opt/musicmouse`.
+- Installs a pinned `uv` release and uses it to create `/opt/musicmouse/.venv` with
+  Python `pi_musicmouse_python_version` (the backend needs 3.12+ for PEP 695 syntax;
+  Raspbian Bookworm's apt python3 is only 3.11) and `pip install` the backend into it.
+- Builds the frontend (`web/`) on the *control machine* (delegated to `localhost`, not
+  the Pi - no Node/npm is installed on the Pi) and syncs `web/dist` over.
+- Installs `/media/musicmouse/config.yml` from `files/config-<inventory_hostname>.yml`
+  (one file per host - the schema differs between a host with a real mouse attached and
+  a display-only one; see those files' own comments) and the systemd unit copied
+  straight from the checked-out repo's `python-backend/musicmouse.service`, so it can
+  never drift from what's actually in the app repo.
+- Sets up a Samba share of `/media/musicmouse` for copying music onto the device.
+
+Manual steps after a first deploy: set the Samba password with `smbpasswd`, and copy
+music into `/media/musicmouse` (or via the Samba share).