home: auto-duck Spotify while speaking on Discord voice

Add a systemd user service (spotify-duck.py) that meters Discord's output
stream and the denoised rnnoise mic via pw-record and lowers Spotify's volume
via wpctl while anyone is speaking, restoring it a beat after silence. Tuning
knobs are exposed as service Environment entries. Ignore Python __pycache__.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

home/zephrynis.nix

@@ -71,6 +71,7 @@ in
   imports = [
     inputs.illogical-flake.homeManagerModules.default
     inputs.spicetify-nix.homeManagerModules.spicetify
+    ./spotify-ducking.nix
   ];
 
   home.username = "zephrynis";