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/dbq/albums.sql.go

@@ -168,6 +168,41 @@ func (q *Queries) GetAlbumCoverageRollup(ctx context.Context) (GetAlbumCoverageR
 	return i, err
 }
 
+const getAlbumWithArtist = `-- name: GetAlbumWithArtist :one
+SELECT albums.id, albums.title, albums.sort_title, albums.artist_id, albums.release_date, albums.mbid, albums.cover_art_path, albums.created_at, albums.updated_at, albums.cover_art_source, albums.cover_art_sources_version, artists.name AS artist_name
+FROM albums
+JOIN artists ON artists.id = albums.artist_id
+WHERE albums.id = $1
+`
+
+type GetAlbumWithArtistRow struct {
+	Album      Album
+	ArtistName string
+}
+
+// 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.
+func (q *Queries) GetAlbumWithArtist(ctx context.Context, id pgtype.UUID) (GetAlbumWithArtistRow, error) {
+	row := q.db.QueryRow(ctx, getAlbumWithArtist, id)
+	var i GetAlbumWithArtistRow
+	err := row.Scan(
+		&i.Album.ID,
+		&i.Album.Title,
+		&i.Album.SortTitle,
+		&i.Album.ArtistID,
+		&i.Album.ReleaseDate,
+		&i.Album.Mbid,
+		&i.Album.CoverArtPath,
+		&i.Album.CreatedAt,
+		&i.Album.UpdatedAt,
+		&i.Album.CoverArtSource,
+		&i.Album.CoverArtSourcesVersion,
+		&i.ArtistName,
+	)
+	return i, err
+}
+
 const getAlbumsByIDs = `-- name: GetAlbumsByIDs :many
 SELECT id, title, sort_title, artist_id, release_date, mbid, cover_art_path, created_at, updated_at, cover_art_source, cover_art_sources_version FROM albums WHERE id = ANY($1::uuid[])
 `
@@ -389,6 +424,56 @@ func (q *Queries) ListAlbumsByArtist(ctx context.Context, artistID pgtype.UUID)
 	return items, nil
 }
 
+const listAlbumsByArtistWithTrackCount = `-- name: ListAlbumsByArtistWithTrackCount :many
+SELECT albums.id, albums.title, albums.sort_title, albums.artist_id, albums.release_date, albums.mbid, albums.cover_art_path, albums.created_at, albums.updated_at, albums.cover_art_source, albums.cover_art_sources_version,
+       (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
+`
+
+type ListAlbumsByArtistWithTrackCountRow struct {
+	Album      Album
+	TrackCount int64
+}
+
+// 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.
+func (q *Queries) ListAlbumsByArtistWithTrackCount(ctx context.Context, artistID pgtype.UUID) ([]ListAlbumsByArtistWithTrackCountRow, error) {
+	rows, err := q.db.Query(ctx, listAlbumsByArtistWithTrackCount, artistID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []ListAlbumsByArtistWithTrackCountRow
+	for rows.Next() {
+		var i ListAlbumsByArtistWithTrackCountRow
+		if err := rows.Scan(
+			&i.Album.ID,
+			&i.Album.Title,
+			&i.Album.SortTitle,
+			&i.Album.ArtistID,
+			&i.Album.ReleaseDate,
+			&i.Album.Mbid,
+			&i.Album.CoverArtPath,
+			&i.Album.CreatedAt,
+			&i.Album.UpdatedAt,
+			&i.Album.CoverArtSource,
+			&i.Album.CoverArtSourcesVersion,
+			&i.TrackCount,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const listAlbumsByGenre = `-- name: ListAlbumsByGenre :many
 SELECT DISTINCT ON (albums.id) albums.id, albums.title, albums.sort_title, albums.artist_id, albums.release_date, albums.mbid, albums.cover_art_path, albums.created_at, albums.updated_at, albums.cover_art_source, albums.cover_art_sources_version
 FROM albums

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;