fix(recommendation): artist-level arms take one track per related artist in turn (#5297)

The similar_artists and coplay_artists arms ordered by artist score, so the
closest related artist's catalogue filled the whole LIMIT. On the operator's
library the 30-row similar_artists arm held exactly one artist for all 17
seeds measured (#3879), though each seed had 7-33 similar artists in the
library. Both arms now rank tracks within each artist and take every
artist's first track, best artist first, before anyone's second.

Both also skip missing tracks: the outer select already dropped them, but
only after they had taken places in the arm's LIMIT.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/dbq/recommendation.sql.go

@@ -828,13 +828,23 @@ lb_similar AS (
     LIMIT $5
 ),
 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 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, md5(t.id::text || $12::text)
+    -- Round-robin across the similar artists (#5297): each artist's first
+    -- track, best-scoring artist first, then each one's second, and so on.
+    -- Ordering by artist score alone let the closest similar artist's
+    -- catalogue fill the whole LIMIT; on the operator's library this arm held
+    -- exactly one artist for all 17 seeds measured (#3879).
+    SELECT track_id, sim_score
+    FROM (
+        SELECT t.id AS track_id, asim.score * 0.5 AS sim_score, asim.score AS artist_score,
+               row_number() OVER (PARTITION BY asim.artist_b_id
+                                  ORDER BY md5(t.id::text || $12::text)) AS artist_turn
+        FROM artist_similarity asim
+        JOIN tracks t ON t.artist_id = asim.artist_b_id AND t.missing_since IS NULL
+        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)
+    ) ranked
+    ORDER BY artist_turn, artist_score DESC, md5(track_id::text || $12::text)
     LIMIT $6
 ),
 tag_overlap AS (
@@ -882,14 +892,20 @@ coplay_artists AS (
     -- the coplay worker). Mirrors similar_artists but from local co-occurrence
     -- instead of ListenBrainz; empty on single-user servers. Same 0.5 damp as
     -- similar_artists since it's artist-level.
-    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 seed_artist sa ON asim.artist_a_id = sa.artist_id
-    WHERE asim.source = 'user_cooccurrence'
-      AND t.id NOT IN (SELECT id FROM excluded_ids)
-      AND t.id <> $2
-    ORDER BY asim.score DESC, md5(t.id::text || $12::text)
+    -- Round-robin across artists for the same reason as similar_artists.
+    SELECT track_id, sim_score
+    FROM (
+        SELECT t.id AS track_id, asim.score * 0.5 AS sim_score, asim.score AS artist_score,
+               row_number() OVER (PARTITION BY asim.artist_b_id
+                                  ORDER BY md5(t.id::text || $12::text)) AS artist_turn
+        FROM artist_similarity asim
+        JOIN tracks t ON t.artist_id = asim.artist_b_id AND t.missing_since IS NULL
+        JOIN seed_artist sa ON asim.artist_a_id = sa.artist_id
+        WHERE asim.source = 'user_cooccurrence'
+          AND t.id NOT IN (SELECT id FROM excluded_ids)
+          AND t.id <> $2
+    ) ranked
+    ORDER BY artist_turn, artist_score DESC, md5(track_id::text || $12::text)
     LIMIT $11
 ),
 random_fill AS (

internal/db/queries/recommendation.sql

@@ -96,13 +96,23 @@ lb_similar AS (
     LIMIT $5
 ),
 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 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, md5(t.id::text || $12::text)
+    -- Round-robin across the similar artists (#5297): each artist's first
+    -- track, best-scoring artist first, then each one's second, and so on.
+    -- Ordering by artist score alone let the closest similar artist's
+    -- catalogue fill the whole LIMIT; on the operator's library this arm held
+    -- exactly one artist for all 17 seeds measured (#3879).
+    SELECT track_id, sim_score
+    FROM (
+        SELECT t.id AS track_id, asim.score * 0.5 AS sim_score, asim.score AS artist_score,
+               row_number() OVER (PARTITION BY asim.artist_b_id
+                                  ORDER BY md5(t.id::text || $12::text)) AS artist_turn
+        FROM artist_similarity asim
+        JOIN tracks t ON t.artist_id = asim.artist_b_id AND t.missing_since IS NULL
+        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)
+    ) ranked
+    ORDER BY artist_turn, artist_score DESC, md5(track_id::text || $12::text)
     LIMIT $6
 ),
 tag_overlap AS (
@@ -150,14 +160,20 @@ coplay_artists AS (
     -- the coplay worker). Mirrors similar_artists but from local co-occurrence
     -- instead of ListenBrainz; empty on single-user servers. Same 0.5 damp as
     -- similar_artists since it's artist-level.
-    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 seed_artist sa ON asim.artist_a_id = sa.artist_id
-    WHERE asim.source = 'user_cooccurrence'
-      AND t.id NOT IN (SELECT id FROM excluded_ids)
-      AND t.id <> $2
-    ORDER BY asim.score DESC, md5(t.id::text || $12::text)
+    -- Round-robin across artists for the same reason as similar_artists.
+    SELECT track_id, sim_score
+    FROM (
+        SELECT t.id AS track_id, asim.score * 0.5 AS sim_score, asim.score AS artist_score,
+               row_number() OVER (PARTITION BY asim.artist_b_id
+                                  ORDER BY md5(t.id::text || $12::text)) AS artist_turn
+        FROM artist_similarity asim
+        JOIN tracks t ON t.artist_id = asim.artist_b_id AND t.missing_since IS NULL
+        JOIN seed_artist sa ON asim.artist_a_id = sa.artist_id
+        WHERE asim.source = 'user_cooccurrence'
+          AND t.id NOT IN (SELECT id FROM excluded_ids)
+          AND t.id <> $2
+    ) ranked
+    ORDER BY artist_turn, artist_score DESC, md5(track_id::text || $12::text)
     LIMIT $11
 ),
 random_fill AS (