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/librespot/Dockerfile

@@ -0,0 +1,51 @@
+# Librespot (Spotify Connect) for ARM64
+# Multi-stage: cross-compile Rust app on x86_64 builder, ship slim runtime.
+# Audio output: PulseAudio (connect to audio-bridge via TCP localhost:4713).
+
+FROM rust:1.85.1-bookworm AS builder
+
+# Cross-compile toolchain for aarch64
+RUN rustup target add aarch64-unknown-linux-gnu
+RUN dpkg --add-architecture arm64 \
+ && apt-get update \
+ && apt-get install -y --no-install-recommends \
+      gcc-aarch64-linux-gnu \
+      libpulse-dev:arm64 \
+      libasound2-dev:arm64 \
+      libssl-dev:arm64 \
+ && rm -rf /var/lib/apt/lists/*
+
+# Build librespot. Pin to a known tag.
+ARG LIBRESPOT_VERSION=v0.8.0
+RUN git clone --depth 1 --branch ${LIBRESPOT_VERSION} \
+        https://github.com/librespot-org/librespot.git /src
+WORKDIR /src
+ENV CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER=aarch64-linux-gnu-gcc
+RUN cargo build --release --target aarch64-unknown-linux-gnu --no-default-features --features="pulseaudio-backend"
+
+FROM debian:bookworm-slim
+
+RUN dpkg --add-architecture arm64 \
+ && apt-get update \
+ && apt-get install -y --no-install-recommends \
+      ca-certificates \
+      libpulse0:arm64 \
+      libasound2:arm64 \
+      libssl3:arm64 \
+ && rm -rf /var/lib/apt/lists/*
+
+COPY --from=builder /src/target/aarch64-unknown-linux-gnu/release/librespot /usr/local/bin/librespot
+
+# Cache dir for Spotify credentials
+VOLUME ["/var/cache/librespot"]
+
+# PulseAudio bridge is at localhost:4713 (audio-bridge)
+ENV PULSE_SERVER=tcp:127.0.0.1:4713
+
+CMD ["librespot", \
+     "--name", "Klubhaus", \
+     "--backend", "pulseaudio", \
+     "--device", "alsa_output.default", \
+     "--bitrate", "320", \
+     "--enable-audio-cache", \
+     "--cache", "/var/cache/librespot"]