feat: GnuDB fallback with retries when abcde CDDB lookup returns nothing

- New module cddb.py: direct GnuDB/FreeDB HTTP lookup using CDDB protocol,
  with same retry+random-delay logic as MusicBrainz barcode lookup
- get_discid() reads disc fingerprint via cd-discid before ripping
- If abcde returns no CDDB track data, lookup_by_discid() queries GnuDB
  directly (up to 3 retries, 2-6 s random pause between attempts)
- TrackInfo moved from ripper.py to models.py to break circular import
  (cddb.py and ripper.py both use TrackInfo)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_ripper.py

@@ -4,10 +4,9 @@ from pathlib import Path
 from unittest.mock import MagicMock, patch
 
 from musiksammlung.config import AudioFormat
-from musiksammlung.models import Album, Disc, Track
+from musiksammlung.models import Album, Disc, Track, TrackInfo
 from musiksammlung.ripper import (
     RipperConfig,
-    TrackInfo,
     _clean_input,
     _extract_tracks,
     _parse_cddb_lines,