Blend taste profile with text-embedded mood descriptions (e.g. "chill
ambient lo-fi") using pre-blended vector search against the existing
HNSW index. New optional `vibe` and `alpha` params on playlist generate
and recommendations endpoints. Backward compatible — no vibe = pure
taste profile (alpha=1.0).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
