feat(server): similar-artists + per-user artist top-tracks endpoints

GET /api/artists/{id}/similar — in-library artists ranked by similarity
score (deduped across sources), ArtistRef list with cover + album count.
GET /api/artists/{id}/top-tracks — current user's most-played tracks for
the artist (skips excluded, quarantine filtered).

internal/db/queries/recommendation.sql

@@ -231,6 +231,32 @@ WHERE NOT EXISTS (
 ORDER BY p.cnt DESC, t.id
 LIMIT $2;
 
+-- name: ListMostPlayedTracksForArtist :many
+-- Top tracks for one artist by this user's completed-play count (skips
+-- excluded, quarantine filtered). Same projection as
+-- ListMostPlayedTracksForUser plus an artist_id filter, so the handler
+-- reuses trackRefFrom(row.Track, row.AlbumTitle, row.ArtistName).
+WITH plays AS (
+    SELECT track_id, count(*) AS cnt
+    FROM play_events
+    WHERE user_id = sqlc.arg(user_id) AND was_skipped = false
+    GROUP BY track_id
+)
+SELECT sqlc.embed(t),
+       albums.title AS album_title,
+       artists.name AS artist_name
+FROM plays p
+JOIN tracks t   ON t.id = p.track_id
+JOIN albums     ON albums.id  = t.album_id
+JOIN artists    ON artists.id = t.artist_id
+WHERE t.artist_id = sqlc.arg(artist_id)
+  AND NOT EXISTS (
+      SELECT 1 FROM lidarr_quarantine q
+      WHERE q.user_id = sqlc.arg(user_id) AND q.track_id = t.id
+  )
+ORDER BY p.cnt DESC, t.id
+LIMIT sqlc.arg(result_limit);
+
 -- name: ListLastPlayedArtistsForUser :many
 -- M6a: artists ranked by max(play_events.started_at) for the user, with
 -- a derived cover_album_id via a representative-album lateral join (most