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/db/queries/recommendation.sql

@@ -36,9 +36,13 @@ WHERE t.id <> $2
 -- Returns same shape as LoadRadioCandidates plus similarity_score column.
 
 WITH
-seed_info AS (
-    SELECT t.artist_id,
-           trim(g)::text AS tag
+seed_artist AS (
+    SELECT artist_id
+    FROM tracks
+    WHERE tracks.id = $2
+),
+seed_tags AS (
+    SELECT trim(g) AS tag
     FROM tracks t
     LEFT JOIN LATERAL regexp_split_to_table(coalesce(t.genre, ''), '[;,]') AS g(tag) ON true
     WHERE t.id = $2 AND trim(g) <> ''
@@ -63,7 +67,7 @@ similar_artists AS (
     SELECT t.id AS track_id, asim.score * 0.5 AS sim_score
     FROM artist_similarity asim
     JOIN tracks t ON t.artist_id = asim.artist_b_id
-    JOIN (SELECT DISTINCT artist_id FROM seed_info) sa ON asim.artist_a_id = sa.artist_id
+    JOIN seed_artist sa ON asim.artist_a_id = sa.artist_id
     WHERE asim.source = 'listenbrainz'
       AND t.id NOT IN (SELECT id FROM excluded_ids)
     ORDER BY asim.score DESC, random()
@@ -72,10 +76,10 @@ similar_artists AS (
 tag_overlap AS (
     SELECT t.id AS track_id,
            (count(DISTINCT trim(g))::float8
-            / GREATEST((SELECT count(DISTINCT tag) FROM seed_info), 1)) AS sim_score
+            / GREATEST((SELECT count(*) FROM seed_tags), 1)) AS sim_score
     FROM tracks t
     LEFT JOIN LATERAL regexp_split_to_table(coalesce(t.genre, ''), '[;,]') AS g_split(g) ON true
-    WHERE trim(g_split.g) IN (SELECT tag FROM seed_info)
+    WHERE trim(g_split.g) IN (SELECT tag FROM seed_tags)
       AND t.id NOT IN (SELECT id FROM excluded_ids)
       AND t.id <> $2
     GROUP BY t.id
@@ -92,7 +96,7 @@ likes_overlap AS (
           SELECT 1 FROM tracks t
           LEFT JOIN LATERAL regexp_split_to_table(coalesce(t.genre, ''), '[;,]') AS g_overlap(g) ON true
           WHERE t.id = gl.track_id
-            AND trim(g_overlap.g) IN (SELECT tag FROM seed_info)
+            AND trim(g_overlap.g) IN (SELECT tag FROM seed_tags)
       )
     ORDER BY random()
     LIMIT $8