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/compose.yaml

@@ -78,4 +78,71 @@ services:
       FLASH_DURATION_SECONDS: "14"
       TZ: America/Los_Angeles
 
+  # Audio router: PipeWire + WirePlumber + PulseAudio bridge + BlueZ.
+  # Routes audio from librespot / mopidy / phone-Bluetooth to USB DAC (Q5K)
+  # via ALSA. Falls back to RPi 3.5mm / HDMI if no USB DAC.
+  # PEQ is applied on the DAC side (Q5K's Qudelix app, 20 bands), not here.
+  audio-bridge:
+    build: ./audio-bridge
+    container_name: audio-bridge
+    restart: unless-stopped
+    privileged: true
+    network_mode: host
+    devices:
+      - /dev/snd:/dev/snd
+    volumes:
+      - /run/dbus:/run/dbus                # system DBus (bluetoothd, wireplumber)
+      - /run/user/0:/run/user/0            # PipeWire runtime dir
+    cap_add:
+      - SYS_ADMIN                          # bluetoothd needs CAP_SYS_ADMIN for HCI
+      - NET_ADMIN
+
+  # Spotify Connect: phone's Spotify app shows "Klubhaus" as a Connect target.
+  # Outputs audio to audio-bridge via PulseAudio (localhost:4713).
+  # Built locally on RPi (cross-compile is too slow under qemu).
+  librespot:
+    build: ./librespot
+    image: infra-librespot:latest
+    container_name: librespot
+    restart: unless-stopped
+    depends_on:
+      - audio-bridge
+    network_mode: host
+    command: >
+      --name "Klubhaus"
+      --backend pulseaudio
+      --bitrate 320
+      --initial-volume 100
+      --enable-audio-cache
+      --cache /var/cache/librespot
+    volumes:
+      - librespot_cache:/var/cache/librespot
+
+  # Mopidy: Qobuz + local files + internet radio. Web UI on http://rpi:6680.
+  # Outputs audio to audio-bridge via PulseAudio (localhost:4713).
+  # Get Qobuz app_id + secret from https://developer.qobuz.com/api/v1/oauth
+  # Built locally from python:3.12-slim + pip install (no ghcr.io image).
+  mopidy:
+    build: ./mopidy
+    image: infra-mopidy:latest
+    container_name: mopidy
+    restart: unless-stopped
+    depends_on:
+      - audio-bridge
+    network_mode: host
+    ports:
+      - "6680:6680"
+    volumes:
+      - ./mopidy/data:/var/lib/mopidy
+      - ./mopidy/music:/music              # drop FLACs in infra/mopidy/music/
+    environment:
+      TZ: America/Los_Angeles
+      QOBUZ_APP_ID: "${QOBUZ_APP_ID:-}"
+      QOBUZ_APP_SECRET: "${QOBUZ_APP_SECRET:-}"
+      QOBUZ_USERNAME: "${QOBUZ_USERNAME:-}"
+      QOBUZ_PASSWORD: "${QOBUZ_PASSWORD:-}"
+
+volumes:
+  librespot_cache:
+