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

internal/db/migrations/0012_artist_similarity_unmatched.down.sql

@@ -0,0 +1,2 @@
+DROP INDEX IF EXISTS artist_similarity_unmatched_seed_score_idx;
+DROP TABLE IF EXISTS artist_similarity_unmatched;

internal/db/migrations/0012_artist_similarity_unmatched.up.sql

@@ -0,0 +1,17 @@
+-- M5c: persist unmatched-similar-artist MBIDs that the M4b worker would
+-- otherwise discard. Mirrors artist_similarity shape: same composite PK
+-- with source, same (seed_id, score DESC) index, same source enum check.
+-- The candidate side is text + name (no FK) — that's the whole point.
+
+CREATE TABLE artist_similarity_unmatched (
+    seed_artist_id  uuid              NOT NULL REFERENCES artists(id) ON DELETE CASCADE,
+    candidate_mbid  text              NOT NULL,
+    candidate_name  text              NOT NULL,
+    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 (seed_artist_id, candidate_mbid, source)
+);
+
+CREATE INDEX artist_similarity_unmatched_seed_score_idx
+    ON artist_similarity_unmatched (seed_artist_id, score DESC);