feat(db): add M6a library-list + artist-tracks queries

Appends ListArtistsAlphaWithCovers, ListAlbumsAlphaWithArtist,
CountAlbums, and ListArtistTracksForUser queries; regenerates sqlc.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/db/queries/artists.sql

@@ -42,3 +42,25 @@ SELECT COUNT(*) FROM artists WHERE name ILIKE '%' || $1::text || '%';
 -- Batched lookup used by M5c suggestion attribution to resolve top-3
 -- contributing seed UUIDs back to artist names in one round-trip.
 SELECT * FROM artists WHERE id = ANY($1::uuid[]);
+
+-- name: ListArtistsAlphaWithCovers :many
+-- M6a: alpha-sorted artist list with derived cover_album_id (most-recent
+-- album whose cover_art_path is set). Replaces ListArtistsAlpha for the
+-- /api/artists?sort=alpha branch — the new column is appended, so the
+-- alpha branch is purely additive on the wire. album_count is computed
+-- in the same query to drop the old N+1 in handleListArtists.
+SELECT sqlc.embed(artists),
+       cov.id AS cover_album_id,
+       cnt.album_count::bigint AS album_count
+FROM artists
+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 artists.sort_name, artists.name, artists.id
+LIMIT $1 OFFSET $2;