audio: add PipeWire+BT audio-bridge, mopidy (Qobuz+local), librespot (Spotify Connect)

- audio-bridge: alpine + pipewire + pipewire-pulse + wireplumber + bluez + dbus + tini
  Routes audio from librespot/mopidy/phone-BT to USB DAC (Q5K) via ALSA.
  Privileged + host network for bluetoothd HCI access.
- mopidy: built from python:3.12-slim-bookworm + pip (no ghcr.io dep).
  Qobuz (auth from developer.qobuz.com) + local files. PA bridge to audio-bridge.
- librespot: built from rust:1.85.1 on RPi (cross-compile too slow under qemu).
  Spotify Connect target named 'Klubhaus'. PA bridge to audio-bridge.

Both mopidy and librespot build locally to avoid ghcr.io auth/rate-limit issues.
PEQ stays on Q5K side (Qudelix app, 20 bands) for source-independent EQ.

infra/audio-bridge/Dockerfile

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+# Audio bridge: PipeWire + PulseAudio bridge + BlueZ
+# Routes audio from librespot / mopidy / phone-Bluetooth to USB DAC (Qudelix 5K)
+# Cross-built for linux/arm64 on x86_64 build host via buildx + qemu.
+
+FROM alpine:3.20
+
+# PipeWire = audio router, handles ALSA sink + bluetooth A2DP sink + PA bridge.
+# wireplumber = PipeWire session manager (loads bluetooth modules on demand).
+# bluez = Bluetooth stack (needed for "bluetooth target" — phone → Pi audio).
+RUN apk add --no-cache \
+    pipewire \
+    pipewire-pulse \
+    wireplumber \
+    bluez \
+    alsa-utils \
+    alsa-plugins-pulse \
+    dbus \
+    tini
+
+COPY entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
+
+# D-Bus socket dir for bluetoothd / wireplumber / pipewire
+VOLUME ["/run/dbus", "/run/user/0"]
+
+ENTRYPOINT ["/sbin/tini", "--", "/entrypoint.sh"]

infra/audio-bridge/entrypoint.sh

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+#!/bin/sh
+# audio-bridge entrypoint
+# Starts DBus, BlueZ, WirePlumber, then PipeWire. PA clients (librespot, mopidy)
+# connect via pipewire-pulse bridge on TCP localhost:4713.
+
+set -e
+
+mkdir -p /run/dbus /run/user/0
+chmod 0755 /run/dbus
+chmod 0700 /run/user/0
+
+# System DBus (needed by bluetoothd, wireplumber, pipewire)
+dbus-daemon --system --fork
+sleep 0.5
+
+# BlueZ (Bluetooth stack). --compat = legacy mode for older audio devices.
+# Disabled if no /dev/bluetooth/rfkill — wireplumber will skip BT modules.
+if [ -d /sys/class/bluetooth ]; then
+    bluetoothd --compat --nodetach &
+    sleep 0.5
+    echo "[audio-bridge] bluetoothd started"
+else
+    echo "[audio-bridge] no bluetooth hardware — skipping bluetoothd"
+fi
+
+# PipeWire runtime env
+export XDG_RUNTIME_DIR=/run/user/0
+export DBUS_SYSTEM_BUS_ADDRESS=unix:path=/run/dbus/system_bus_socket
+
+# WirePlumber first (it manages PipeWire session/policy, loads BT modules)
+wireplumber &
+sleep 1
+
+# PipeWire last. The pipewire-pulse package auto-loads the PA bridge module
+# so PA clients (librespot, mopidy) can connect via TCP localhost:4713.
+exec pipewire