perf(api): collapse N+1 in /api/artists/{id} + 1 round-trip in /api/albums/{id}

Two new sqlc queries replace three sequential per-album round trips
that were dominating detail-screen latency.

GetAlbumWithArtist: handleGetAlbum was doing GetAlbumByID then
GetArtistByID — separate round trips for one logical lookup. The new
query joins albums + artists with sqlc.embed and returns both in one
SELECT. Detail-page DB cost: 3 trips → 2.

ListAlbumsByArtistWithTrackCount: handleGetArtist was loading the
artist's album list, then issuing one CountTracksByAlbum per album to
populate track_count. On a 30-album artist that's 32 sequential
queries — each ~5ms over a local DB, ~30ms over a remote one. The
new query embeds the album row + a correlated count(*) subquery, so
every album's track count comes back in one SELECT regardless of
album count. Detail-page DB cost: 1 + N → 1 + 1.

Together these account for the bulk of cold-cache navigation latency
on the Flutter client. Combined with the existing SWR + nav
hydration on the client side, detail screens should render their
header instantly and the body within one round trip instead of
N+constant.

internal/db/queries/albums.sql

@@ -14,6 +14,26 @@ RETURNING *;
 -- name: GetAlbumByID :one
 SELECT * FROM albums WHERE id = $1;
 
+-- name: GetAlbumWithArtist :one
+-- Combined fetch for /api/albums/{id}: returns the album row + the
+-- joined artist name in a single round trip. Replaces a sequential
+-- GetAlbumByID + GetArtistByID pair on the hot detail-page path.
+SELECT sqlc.embed(albums), artists.name AS artist_name
+FROM albums
+JOIN artists ON artists.id = albums.artist_id
+WHERE albums.id = $1;
+
+-- name: ListAlbumsByArtistWithTrackCount :many
+-- Replaces the N+1 pattern in handleGetArtist (1 + N CountTracksByAlbum
+-- per album). Returns each album joined with its track count via a
+-- correlated subquery — single round trip regardless of album count.
+SELECT sqlc.embed(albums),
+       (SELECT count(*) FROM tracks t WHERE t.album_id = albums.id)::bigint
+           AS track_count
+FROM albums
+WHERE albums.artist_id = $1
+ORDER BY release_date NULLS LAST, sort_title;
+
 -- name: GetAlbumByArtistAndTitle :one
 -- Scanner uses this for the no-mbid dedupe path: resolve-or-create.
 SELECT * FROM albums WHERE artist_id = $1 AND title = $2 LIMIT 1;