Replaces the Soulseek-only bit-depth heuristic with a source-agnostic
quality system that works across all download sources.
## core/quality/model.py (new)
- AudioQuality dataclass: format, bitrate, sample_rate, bit_depth
- QualityTarget: one ranked entry in the user's priority list
- filter_and_rank(): source-neutral candidate ranking
- rank_candidate(): scores any AudioQuality against ranked_targets
- v2_qualities_to_ranked_targets(): migration helper
## core/download_plugins/types.py
- SearchResult gains sample_rate + bit_depth fields
- audio_quality property returns unified AudioQuality
- AlbumResult gets audio_quality aggregated from tracks
## core/soulseek_client.py
- Parses slskd attributes array (type 4=sample_rate, type 5=bit_depth)
- Real values instead of kbps heuristic
- filter_results_by_quality_preference() replaced by filter_and_rank()
## database/music_database.py
- Quality profile v3 with ranked_targets list
- Auto-migration v2 → v3 on load
- Presets (audiophile/balanced/space_saver) updated to v3
## core/imports/file_ops.py
- probe_audio_quality(): reads actual downloaded file via mutagen
returns AudioQuality with ground-truth values
## core/imports/guards.py
- check_quality_target(): replaces check_flac_bit_depth
checks all formats/sources against ranked_targets
- check_flac_bit_depth() kept as backwards-compat wrapper
## core/imports/pipeline.py
- Uses check_quality_target() instead of check_flac_bit_depth()
- Quality mismatch triggers _requeue_quarantined_task_for_retry('quality')
so next-best candidate is tried before failing (same as AcoustID)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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