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/files/config-musicdolphin.yml

@@ -0,0 +1,29 @@
+---
+# musicdolphin: display-only instance. No physical mouse attached (for
+# now), so serial and audio are both "simulate" - the web front-end (shown locally in
+# the pi_kiosk role's Firefox kiosk window, and reachable on the LAN for a tablet) is a
+# complete way to drive the player on its own. `figures` still needs at least one entry
+# (the schema requires it) even though nothing will ever scan a tag here.
+general:
+  library:
+    root: /media/musicmouse
+
+  serial_port: "simulate"
+  alsa_device: "simulate"
+
+  web:
+    host: "0.0.0.0"
+    port: 8080
+    static_dir: /opt/musicmouse/web/dist
+
+  button_leds_brightness: 0.5
+  volume_increment: 5
+  min_volume: 0
+  max_volume: 100
+
+figures:
+  # Placeholder - serial_port is "simulate" so no reader ever reports a tag id, but the
+  # schema requires at least one figure to be configured.
+  placeholder:
+    id: "0000000001"
+    colors: ["#ffffff", "#ffffff", "#000000", "#ffffff"]