fix(db): split seed_info CTE so similar-artist works without seed genre

The combined seed_info CTE filtered the seed out when its genre was
NULL/empty. Restored the spec's two-CTE design (seed_artist + seed_tags)
so similar-artist matching works regardless of the seed's genre coverage.
Drops the genre workaround from the SimilarArtist test.

internal/recommendation/candidates_v2_test.go

@@ -73,11 +73,6 @@ func TestLoadCandidatesFromSimilarity_LBSimilarSourceContributes(t *testing.T) {
 func TestLoadCandidatesFromSimilarity_SimilarArtistTracksContribute(t *testing.T) {
 	f := newFixture(t, 1) // creates 1 artist + 1 album + 1 track (the seed)
 	seed := f.tracks[0]
-	// seed_info CTE only emits artist_id rows when genre is non-NULL (it
-	// uses a LATERAL regexp_split_to_table join filtered by trim(g) <> '').
-	// Give the seed a genre so that seed_info is populated and the
-	// similar_artists JOIN can find artist_a_id = seed.ArtistID.
-	helperSetTrackGenre(t, f, seed.ID, "Rock")
 	// Add a SECOND artist + track in that artist; relate the two artists via artist_similarity.
 	otherArtist, _ := f.q.UpsertArtist(context.Background(), dbq.UpsertArtistParams{Name: "OtherArtist", SortName: "OtherArtist"})
 	otherAlbum, _ := f.q.UpsertAlbum(context.Background(), dbq.UpsertAlbumParams{Title: "OtherAlbum", SortTitle: "OtherAlbum", ArtistID: otherArtist.ID})