Adds an explicit field to the Album dataclass in core/metadata/types.py
and the client-level Album dataclasses in deezer_client.py,
itunes_client.py, and hydrabase_client.py (the legacy discography path
reads from client objects, not typed dicts).
Deezer extracts explicit_lyrics (int→bool), iTunes extracts
collectionExplicitness ('explicit' string), Hydrabase forwards the
explicit field from the server response. Spotify, Discogs, MusicBrainz,
Qobuz, and Tidal have no explicit signal and stay None.
The flag threads through both builder functions in discography.py and
renders as a small "E" badge next to explicit titles in the discography
download modal and artist-detail page cards.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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