feat(explore): sample the one-album-owned shelf instead of ranking it

"More from artists you own one album by" drew its artists and their
albums most-popular first, so it was a second leaderboard: the same
handful of big names every time the page opened, which is not what the
shelf is saying.  The pool is still bounded, but which artists and
which albums fill it is RANDOM(), so each visit is a different sample.
The test asserts the set rather than the order.

Refs #264

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017HJiuc3ZZhxsPXz3ozTirT

backend/explore/shelves.go

@@ -383,9 +383,13 @@ func (si *SearchIndex) topByPopularity(
 // MusicBrainz IDs, so this never touches the library tables and asks
 // one query rather than one per artist.
 //
-// The artists are drawn most-popular-owned-album first, so a large
-// library's pool is the part of it the user is likeliest to recognise
-// rather than whichever artists sort first.
+// **The row is drawn at random, and that is the whole point of it.**
+// Ordered by popularity it was a second leaderboard: the same handful of
+// big names appeared every time the page opened, which is not what "you
+// own one album by these artists" is saying. The pool is still bounded
+// to `pool` artists — a 4 000-artist library does not need all of them
+// ranked — but which of them, and which of their albums, is `RANDOM()`,
+// so the shelf is a different sample each visit.
 func (si *SearchIndex) unownedAlbumsBySinglyOwnedArtists(
 	ctx context.Context,
 	pool, limit int,
@@ -396,15 +400,17 @@ func (si *SearchIndex) unownedAlbumsBySinglyOwnedArtists(
 		 WHERE entity_type = 2 /* release_group */
 		   AND in_library = 0
 		   AND artist_mbid IN (
-		       SELECT artist_mbid FROM explore_index
-		       WHERE entity_type = 2 /* release_group */
-		         AND in_library = 1
-		         AND artist_mbid != x''
-		       GROUP BY artist_mbid
-		       HAVING COUNT(*) = 1
-		       ORDER BY MAX(popularity) DESC
+		       SELECT artist_mbid FROM (
+		           SELECT artist_mbid FROM explore_index
+		           WHERE entity_type = 2 /* release_group */
+		             AND in_library = 1
+		             AND artist_mbid != x''
+		           GROUP BY artist_mbid
+		           HAVING COUNT(*) = 1
+		       )
+		       ORDER BY RANDOM()
 		       LIMIT ?)
-		 ORDER BY popularity DESC
+		 ORDER BY RANDOM()
 		 LIMIT ?`,
 		pool, limit,
 	))

backend/explore/shelves_test.go

@@ -3,6 +3,7 @@ package explore
 import (
 	"context"
 	"log/slog"
+	"sort"
 	"testing"
 
 	"yellowjacket/backend/database"
@@ -202,6 +203,11 @@ func TestShelves_MoreFromOwnedNeedsExactlyOneOwnedAlbum(t *testing.T) {
 		titles = append(titles, album.Title)
 	}
 
+	// The row is a random sample, so the *set* is what is asserted and
+	// not the order — see `unownedAlbumsBySinglyOwnedArtists` for why
+	// the ordering was given up.
+	sort.Strings(titles)
+
 	if len(titles) != 2 || titles[0] != "Second" || titles[1] != "Third" {
 		t.Fatalf("albums = %v, want [Second Third]", titles)
 	}