Add vibe-aware playlists with CLAP text embeddings

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>

alembic/versions/002_add_playlist_vibe.py

@@ -0,0 +1,26 @@
+"""Add vibe and alpha columns to playlists
+
+Revision ID: 002
+Revises: 001
+Create Date: 2026-02-22
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "002"
+down_revision: Union[str, None] = "001"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("playlists", sa.Column("vibe", sa.Text, nullable=True))
+    op.add_column("playlists", sa.Column("alpha", sa.REAL, nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("playlists", "alpha")
+    op.drop_column("playlists", "vibe")