feat(db): add artist_similarity_unmatched schema (migration 0012)

internal/db/queries/similarity.sql

@@ -48,3 +48,15 @@ INSERT INTO artist_similarity (artist_a_id, artist_b_id, score, source, fetched_
 VALUES ($1, $2, $3, 'listenbrainz', now())
 ON CONFLICT (artist_a_id, artist_b_id, source)
 DO UPDATE SET score = EXCLUDED.score, fetched_at = EXCLUDED.fetched_at;
+
+-- name: UpsertArtistSimilarityUnmatched :exec
+-- Persists an out-of-library similar-artist MBID. Idempotent on
+-- (seed_artist_id, candidate_mbid, source) — re-fetches refresh the
+-- name/score and bump fetched_at.
+INSERT INTO artist_similarity_unmatched (
+    seed_artist_id, candidate_mbid, candidate_name, score, source
+) VALUES ($1, $2, $3, $4, $5)
+ON CONFLICT (seed_artist_id, candidate_mbid, source) DO UPDATE SET
+    candidate_name = EXCLUDED.candidate_name,
+    score          = EXCLUDED.score,
+    fetched_at     = now();