Integrates play history data into the discovery algorithm:
- Listening profile: _get_listening_profile() builds user's top artists,
genres, play counts, and listening velocity from the last 30 days
- Artist genre cache: pre-built from local DB for O(1) genre lookups
- Release Radar: +10 genre affinity, +15 artist familiarity, -10 overplay
penalty. Weights rebalanced to 45% recency + 25% popularity + bonuses
- Discovery Weekly: serendipity scoring within tiers — boosts unheard
artists in preferred genres, penalizes overplayed artists
- Recent Albums: adaptive time window (21-60 days) based on listening
velocity — heavy listeners get fresher content, casual listeners more
- New "Because You Listen To" sections: personalized carousels based on
user's top 3 played artists via similar artists + genre fallback
- New endpoint: /api/discover/because-you-listen-to with artist images
- Frontend: BYLT sections with artist photo headers on discover page
- All changes gracefully fall back when no listening data exists
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