Add mpv config with EBU R128 loudness normalization

Adds config/mpv.conf with af=loudnorm filter for consistent volume
across radio stations. No more manual volume adjustment when switching.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config/mpv.conf

@@ -0,0 +1,9 @@
+[pyradio]
+volume=50
+
+[default]
+ytdl-format=bestaudio/worst
+no-video=yes
+
+# EBU R128 loudness normalization — consistent volume across stations
+af=loudnorm=I=-16:LRA=11:TP=-1