Add iTunes and AcoustID as additional free music enrichment sources

- itunes.py: zero-auth fallback for album artwork when MusicBrainz/Cover Art
  Archive has no match — kicks in automatically during music enrichment,
  toggleable in Settings (on by default, no key required).
- acoustid.py: audio fingerprint identification (Chromaprint fpcalc + the
  AcoustID API) for tracks with missing/unknown artist or album — nothing
  to text-search on, so this identifies from the actual audio content
  instead. Requires a free user-supplied API key from acoustid.org/api-key,
  off by default. New /music/identify endpoint + admin trigger button.
- Dockerfile: added libchromaprint-tools for fpcalc.
- Settings page: new Music Metadata & Artwork section with both toggles.

backend/Dockerfile

@@ -1,10 +1,12 @@
 # Kino backend — FastAPI + ffmpeg
 FROM python:3.11-slim
 
-# ffmpeg for HLS transcoding; util-linux for `nice` (already in base but explicit)
+# ffmpeg for HLS transcoding; util-linux for `nice` (already in base but explicit);
+# libchromaprint-tools provides `fpcalc`, used for AcoustID audio fingerprinting.
 RUN apt-get update && apt-get install -y --no-install-recommends \
       ffmpeg \
       curl \
+      libchromaprint-tools \
     && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app