sunken-ship: add ffmpeg to mulbo-server PATH

quality.py's spectral-rolloff probe shells out to ffmpeg to extract
a 30s PCM clip. Without ffmpeg on PATH, subprocess fails silently
and get_or_compute_rolloff returns 0.0 — picker degrades to bitrate
ranking (which is what we were trying to fix). Add ffmpeg via
systemd unit `path = with pkgs; [ ffmpeg ];`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nixos/hosts/sunken-ship.nix

@@ -331,6 +331,10 @@
     after = [ "network-online.target" "navidrome.service" ];
     wants = [ "network-online.target" ];
     wantedBy = [ "multi-user.target" ];
+    # ffmpeg: PCM extraction for quality.py's spectral-rolloff probe
+    # (chromaprint-dupe winner picker in --spectral mode). Without it,
+    # the subprocess silently fails and rolloff returns 0Hz.
+    path = with pkgs; [ ffmpeg ];
     environment = {
       MULBO_UPLOADS_DIR     = "/home/danny/music/mulbo-uploads";
       MULBO_INDEX_DB        = "/var/lib/mulbo-server/index.db";