fix: the taste arm round-robins across taste artists, and tag ties follow the order seed (#3880)

Measured on the operator's library (26,005 tracks, pool scale 2.28): the
arms the library scale grows all have far more rows than it asks for, and
two arms collapsed whatever their limits were.

- taste_overlap ordered by weight then track id, so the strongest taste
  artist's catalogue filled it: 20 tracks of saib. (197 tracks) out of 174
  taste artists, the same lowest-id tracks every time. It now takes one
  track per taste artist, strongest first, md5-seeded, as similar_artists
  does since #5297.
- tag_overlap broke ties in no order. A one-tag seed ties 7,804 tracks,
  and the LIMIT took whichever Postgres reached first, which the order
  seed did not control. Ties now follow md5(id || seed).
- library_scale.go records the per-arm measurement beside the reference
  size it leaves as is.

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

internal/db/queries/recommendation.sql

@@ -138,7 +138,12 @@ tag_overlap AS (
       AND t.id <> $2
     GROUP BY t.id
     HAVING count(DISTINCT trim(g_split.g)) > 0
-    ORDER BY sim_score DESC
+    -- md5 breaks the ties (#3880). A seed with one tag gives every track of
+    -- that genre the same score, 7,804 tracks on the operator's library, and
+    -- with no tie-break the LIMIT took whichever rows Postgres reached first:
+    -- the same slice for every seed of the genre, and not one the order seed
+    -- controls.
+    ORDER BY sim_score DESC, md5(t.id::text || $12::text)
     LIMIT $7
 ),
 likes_overlap AS (
@@ -156,14 +161,25 @@ likes_overlap AS (
     LIMIT $8
 ),
 taste_overlap AS (
-    SELECT t.id AS track_id, 0.0::float8 AS sim_score
-    FROM taste_profile_artists tpa
-    JOIN tracks t ON t.artist_id = tpa.artist_id
-    WHERE tpa.user_id = $1
-      AND tpa.weight > 0
-      AND t.id NOT IN (SELECT id FROM excluded_ids)
-      AND t.id <> $2
-    ORDER BY tpa.weight DESC, t.id
+    -- Round-robin across the taste artists (#3880), as similar_artists is
+    -- (#5297): each artist's first track, strongest artist first, then each
+    -- one's second. Ordered by weight then track id, the strongest artist's
+    -- catalogue filled the whole arm with the same lowest-id tracks every
+    -- time; on the operator's library that was 20 tracks of one artist of
+    -- 174, whose catalogue is 197 tracks.
+    SELECT track_id, sim_score
+    FROM (
+        SELECT t.id AS track_id, 0.0::float8 AS sim_score, tpa.weight,
+               row_number() OVER (PARTITION BY tpa.artist_id
+                                  ORDER BY md5(t.id::text || $12::text)) AS artist_turn
+        FROM taste_profile_artists tpa
+        JOIN tracks t ON t.artist_id = tpa.artist_id AND t.missing_since IS NULL
+        WHERE tpa.user_id = $1
+          AND tpa.weight > 0
+          AND t.id NOT IN (SELECT id FROM excluded_ids)
+          AND t.id <> $2
+    ) ranked
+    ORDER BY artist_turn, weight DESC, md5(track_id::text || $12::text)
     LIMIT $10
 ),
 coplay_artists AS (