audio-bridge: custom pipewire-pulse.conf with TCP 4713 listener

Alpine's pipewire-pulse binary is stripped (-a option removed); the
only way to configure listening address is via config file. Add
/etc/pipewire/pipewire-pulse.conf that overrides server.address to
include tcp:4713 alongside the default unix:native socket.

Server addresses now:
- unix:native (for in-container clients)
- tcp:4713 (for sibling containers connecting via PULSE_SERVER env var)

infra/audio-bridge/pipewire-pulse.conf

@@ -0,0 +1,12 @@
+# Custom PipeWire-Pulse config: override server.address to add TCP 4713 listener.
+# This file is loaded from /etc/pipewire/pipewire-pulse.conf and overrides
+# defaults from /usr/share/pipewire/pipewire-pulse.conf.
+#
+# PA clients (librespot, mopidy) connect via tcp:127.0.0.1:4713.
+
+pulse.properties = {
+    server.address = [
+        "unix:native"
+        "tcp:4713"
+    ]
+}