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/similarity.sql

@@ -60,3 +60,29 @@ ON CONFLICT (seed_artist_id, candidate_mbid, source) DO UPDATE SET
     candidate_name = EXCLUDED.candidate_name,
     score          = EXCLUDED.score,
     fetched_at     = now();
+
+-- name: ListSimilarArtistsForArtist :many
+-- In-library artists similar to a seed artist, ranked by best similarity
+-- score across sources (deduped per candidate). cover_album_id + album_count
+-- mirror ListArtistsAlphaWithCovers so the strip renders identical cards.
+SELECT sqlc.embed(artists),
+       cov.id AS cover_album_id,
+       cnt.album_count::bigint AS album_count
+FROM (
+    SELECT artist_b_id, max(score) AS sim_score
+    FROM artist_similarity
+    WHERE artist_a_id = sqlc.arg(seed_artist_id)
+    GROUP BY artist_b_id
+) s
+JOIN artists ON artists.id = s.artist_b_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 s.sim_score DESC, artists.sort_name
+LIMIT sqlc.arg(result_limit);