feat(db): add migration 0009 for similarity (track_similarity + artist_similarity)

internal/db/migrations/0009_similarity.down.sql

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+DROP TABLE IF EXISTS artist_similarity;
+DROP TABLE IF EXISTS track_similarity;

internal/db/migrations/0009_similarity.up.sql

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+-- M4b: ListenBrainz similarity ingest. Two symmetric tables — tracks and
+-- artists — both keyed by (a_id, b_id, source) so the schema reserves room
+-- for additional similarity sources (musicbrainz_tag, user_cooccurrence)
+-- alongside listenbrainz. M4b only writes 'listenbrainz' rows.
+--
+-- The (a_id, score DESC) index satisfies M4c's hot path: "for seed S, give
+-- me top-N similar tracks/artists descending by score."
+
+CREATE TABLE track_similarity (
+    track_a_id   uuid             NOT NULL REFERENCES tracks(id) ON DELETE CASCADE,
+    track_b_id   uuid             NOT NULL REFERENCES tracks(id) ON DELETE CASCADE,
+    score        DOUBLE PRECISION NOT NULL,
+    source       TEXT             NOT NULL CHECK (source IN ('listenbrainz', 'musicbrainz_tag', 'user_cooccurrence')),
+    fetched_at   timestamptz      NOT NULL DEFAULT now(),
+    PRIMARY KEY (track_a_id, track_b_id, source),
+    CHECK (track_a_id <> track_b_id)
+);
+
+CREATE INDEX track_similarity_a_score_idx
+    ON track_similarity (track_a_id, score DESC);
+
+CREATE TABLE artist_similarity (
+    artist_a_id  uuid             NOT NULL REFERENCES artists(id) ON DELETE CASCADE,
+    artist_b_id  uuid             NOT NULL REFERENCES artists(id) ON DELETE CASCADE,
+    score        DOUBLE PRECISION NOT NULL,
+    source       TEXT             NOT NULL CHECK (source IN ('listenbrainz', 'musicbrainz_tag', 'user_cooccurrence')),
+    fetched_at   timestamptz      NOT NULL DEFAULT now(),
+    PRIMARY KEY (artist_a_id, artist_b_id, source),
+    CHECK (artist_a_id <> artist_b_id)
+);
+
+CREATE INDEX artist_similarity_a_score_idx
+    ON artist_similarity (artist_a_id, score DESC);