fix(recommendation): broaden You-might-like fallback to liked album/track artists (#790)

The fallback pulled artists only from explicit artist-likes (general_likes_artists),
but most users like albums and tracks far more than artists — so the artists row
still came up thin (a couple of tiles) even with a rich library, while the albums
row filled fine.

Broaden both fallbacks to "entities you've shown affinity for":
- artist fallback = explicit artist-likes ∪ artists of liked albums ∪ artists of
  liked tracks.
- album fallback = explicit album-likes ∪ albums of liked tracks.
New dedicated queries (ListYouMightLike{Artist,Album}FallbackForUser) replace the
narrow Rediscover-fallback reuse; same projection so the Go layer still converts
directly. (Aliased + fully-qualified the UNION arms — sqlc merges UNION scopes,
so unqualified user_id was ambiguous across the three like tables.)

Test: 12 liked TRACKS by distinct artists, no artist-likes → the artist row now
fills from their artists (was empty before).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/db/queries/you_might_like.sql

@@ -64,3 +64,62 @@ LEFT JOIN LATERAL (
 WHERE yml.user_id = $1
 ORDER BY yml.rank
 LIMIT $2;
+
+-- name: ListYouMightLikeArtistFallbackForUser :many
+-- Broad "artists you've shown affinity for" pool to fill a thin
+-- You-might-like artists row: explicitly liked artists PLUS the artists of
+-- liked albums and liked tracks. Most users like albums/tracks far more than
+-- they like artists explicitly, so the union is much deeper than
+-- general_likes_artists alone. Daily-stable order; same projection as
+-- ListYouMightLikeArtistsForUser so the Go layer reuses it directly.
+WITH liked_artist_ids AS (
+    SELECT lika.artist_id AS artist_id
+      FROM general_likes_artists lika
+     WHERE lika.user_id = $1
+    UNION
+    SELECT alb.artist_id AS artist_id
+      FROM general_likes_albums likb
+      JOIN albums alb ON alb.id = likb.album_id
+     WHERE likb.user_id = $1
+    UNION
+    SELECT trk.artist_id AS artist_id
+      FROM general_likes likt
+      JOIN tracks trk ON trk.id = likt.track_id
+     WHERE likt.user_id = $1
+)
+SELECT sqlc.embed(artists),
+       cov.id AS cover_album_id,
+       cnt.album_count::bigint AS album_count
+FROM liked_artist_ids lai
+JOIN artists ON artists.id = lai.artist_id
+LEFT JOIN LATERAL (
+    SELECT id FROM albums
+    WHERE artist_id = artists.id AND cover_art_path IS NOT NULL
+    ORDER BY created_at DESC LIMIT 1
+) cov ON true
+LEFT JOIN LATERAL (
+    SELECT count(*) AS album_count
+    FROM albums WHERE artist_id = artists.id
+) cnt ON true
+ORDER BY md5(artists.id::text || current_date::text)
+LIMIT $2;
+
+-- name: ListYouMightLikeAlbumFallbackForUser :many
+-- Broad "albums you've shown affinity for" pool: explicitly liked albums PLUS
+-- the albums of liked tracks. Same projection as ListYouMightLikeAlbumsForUser.
+WITH liked_album_ids AS (
+    SELECT likb.album_id AS album_id
+      FROM general_likes_albums likb
+     WHERE likb.user_id = $1
+    UNION
+    SELECT trk.album_id AS album_id
+      FROM general_likes likt
+      JOIN tracks trk ON trk.id = likt.track_id
+     WHERE likt.user_id = $1
+)
+SELECT sqlc.embed(albums), artists.name AS artist_name
+FROM liked_album_ids lai
+JOIN albums ON albums.id = lai.album_id
+JOIN artists ON artists.id = albums.artist_id
+ORDER BY md5(albums.id::text || current_date::text)
+LIMIT $2;