Add `--primary-only` flag to filter out featured appearances at song level

[shimpeiws]

Problem

GeniusClient.get_artist_songs() returns songs where the target artist may only be featured (primary artist is someone else). Song.artist_id is already captured but not used for filtering.

Proposed Changes

src/barscan/genius/models.py

• Add songs_filtered: int = Field(0) to ArtistWithSongs

src/barscan/genius/client.py

• Add primary_only: bool = False to get_artist_songs() and get_all_lyrics()
• When enabled, filter songs where song.artist_id != artist.id
• Log filtered count at debug level

src/barscan/cli.py

• Add --primary-only option (default off)
• Display filtered count in output

Tests

• tests/test_genius/test_client.py: TestPrimaryOnlyFiltering - mixed artist_id filtering, passthrough when disabled, all-filtered edge case
• tests/test_genius/test_models.py: songs_filtered field
• tests/test_cli/test_commands.py: --primary-only flag

Dependencies

No dependencies.