librespot: add with-libmdns feature for Spotify Connect discovery

Without zeroconf backend, librespot fails with 'Credentials are required
if discovery and oauth login are disabled'. The default features include
with-libmdns but our --no-default-features build excluded it.

Add with-libmdns so the device advertises itself via mDNS and shows up
in phone Spotify apps as a Connect target.

infra/librespot/Dockerfile

@@ -17,9 +17,11 @@ WORKDIR /src
 
 # Limit parallelism to 2 (RPi 3B+ has 4 cores but only 1GB RAM).
 # Lower = less memory pressure, slower build. Higher = OOM risk.
+# Features: pulseaudio (audio out) + native-tls (TLS) + with-libmdns (Spotify Connect
+# discovery via mDNS — required for the device to appear in phone Spotify apps).
 RUN cargo build --release --jobs 2 \
     --no-default-features \
-    --features="pulseaudio-backend,native-tls"
+    --features="pulseaudio-backend,native-tls,with-libmdns"
 
 FROM debian:bookworm-slim