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/mopidy/mopidy.conf

@@ -0,0 +1,38 @@
+[core]
+cache_dir = /var/lib/mopidy/cache
+config_dir = /config
+data_dir = /var/lib/mopidy/data
+max_tracklist_length = 10000
+
+[logging]
+verbosity = info
+format = %(levelname)-8s [%(name)s] %(message)s
+
+# Audio output → PipeWire (via PulseAudio bridge on TCP localhost:4713).
+# PipeWire handles routing to Q5K (USB) or fallback to RPi 3.5mm/HDMI.
+[audio]
+output = pulsesink server=127.0.0.1 name=mopidy
+mixer_volume = 100
+buffer_time = 1000
+
+[http]
+enabled = true
+hostname = 0.0.0.0
+port = 6680
+zeroconf = Klubhaus Mopidy
+
+# Qobuz streaming. Fill in credentials via env vars (set in compose.yaml):
+#   QOBUZ_APP_ID, QOBUZ_APP_SECRET, QOBUZ_USERNAME, QOBUZ_PASSWORD
+[qobuz]
+enabled = true
+app_id = ${QOBUZ_APP_ID}
+app_secret = ${QOBUZ_APP_SECRET}
+username = ${QOBUZ_USERNAME}
+password = ${QOBUZ_PASSWORD}
+
+# Local music files (mount /music in compose)
+[local]
+enabled = true
+media_dirs = /music
+scan_timeout = 5000
+playlists_dir = /var/lib/mopidy/playlists