feat(library): genre + year quick-jumps on album and artist detail — #367

Last bullet of #367. From an album you like, one click to everything else from
that year or in that genre.

Year was free — AlbumRef already carried it. Genre was not: AlbumDetail is
AlbumRef + tracks and neither carried genre, because genre lives on TRACKS. So
both detail responses gained a derived `genres` array, computed from the
entity's tracks rather than stored, since an album's tracks can legitimately
disagree about genre.

Split and trimmed identically to the browse index. That's the invariant this
whole task turned on: if the chip's matching diverged from the index's
splitting, a chip would lead to a page that doesn't contain the album you
clicked from.

No year link on artist detail. An artist spans many years, so a single one
would be a lie about the discography — genres only there.

Genre lookup failure is logged and degrades to no chips rather than failing the
request; a navigation nicety must not 404 a detail page that otherwise loaded.
`genres` is always an array at JSON, never null, matching how every other list
field in this package is emitted.

## Type widening, and the TypeScript version of a lesson from earlier today

Adding a required field to AlbumDetail/ArtistDetail breaks every typed fixture
that constructs one. Six of them across three test files. That's the same shape
as the Go signature changes that cost three CI rounds in #2453 — change a type,
then go find everything that builds it — so I searched for the constructions
before pushing instead of after. All six updated.

Tests: the encoded href for a slash-bearing genre ("Rock/Pop" →
?g=Rock%2FPop), the year href, and the no-tags case rendering no chips at all.
gofmt verified clean via docker rather than guessed.

internal/db/dbq/browse.sql.go

@@ -7,6 +7,8 @@ package dbq
 
 import (
 	"context"
+
+	"github.com/jackc/pgx/v5/pgtype"
 )
 
 const countAlbumsByGenre = `-- name: CountAlbumsByGenre :one
@@ -212,6 +214,69 @@ func (q *Queries) ListAlbumsByYearRangeWithArtist(ctx context.Context, arg ListA
 	return items, nil
 }
 
+const listGenresForAlbum = `-- name: ListGenresForAlbum :many
+SELECT DISTINCT trim(g.genre) AS genre
+FROM tracks
+JOIN LATERAL regexp_split_to_table(coalesce(tracks.genre, ''), '[;,]') AS g(genre) ON true
+WHERE tracks.album_id = $1 AND trim(g.genre) <> ''
+ORDER BY trim(g.genre)
+`
+
+// Distinct genres carried by an album's tracks, for the album detail page's
+// quick-jump chips. Split and trimmed identically to ListGenresWithCount, so a
+// chip always leads to a page that actually contains this album — the two
+// diverging is exactly the bug #367 had to fix in ListAlbumsByGenre.
+func (q *Queries) ListGenresForAlbum(ctx context.Context, albumID pgtype.UUID) ([]string, error) {
+	rows, err := q.db.Query(ctx, listGenresForAlbum, albumID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []string
+	for rows.Next() {
+		var genre string
+		if err := rows.Scan(&genre); err != nil {
+			return nil, err
+		}
+		items = append(items, genre)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
+const listGenresForArtist = `-- name: ListGenresForArtist :many
+SELECT DISTINCT trim(g.genre) AS genre
+FROM tracks
+JOIN LATERAL regexp_split_to_table(coalesce(tracks.genre, ''), '[;,]') AS g(genre) ON true
+WHERE tracks.artist_id = $1 AND trim(g.genre) <> ''
+ORDER BY trim(g.genre)
+`
+
+// Same, across everything by one artist. Alphabetical rather than by count:
+// an artist's genre set is small, and a stable order reads better than a
+// frequency ranking nobody asked about.
+func (q *Queries) ListGenresForArtist(ctx context.Context, artistID pgtype.UUID) ([]string, error) {
+	rows, err := q.db.Query(ctx, listGenresForArtist, artistID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []string
+	for rows.Next() {
+		var genre string
+		if err := rows.Scan(&genre); err != nil {
+			return nil, err
+		}
+		items = append(items, genre)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const listGenresWithCount = `-- name: ListGenresWithCount :many
 SELECT trim(g.genre) AS genre, COUNT(DISTINCT tracks.id)::bigint AS track_count
 FROM tracks

internal/db/queries/browse.sql

@@ -86,3 +86,24 @@ SELECT COUNT(*) FROM albums
 WHERE release_date IS NOT NULL
   AND EXTRACT(YEAR FROM release_date)::int
       BETWEEN sqlc.arg(year_from)::int AND sqlc.arg(year_to)::int;
+
+-- name: ListGenresForAlbum :many
+-- Distinct genres carried by an album's tracks, for the album detail page's
+-- quick-jump chips. Split and trimmed identically to ListGenresWithCount, so a
+-- chip always leads to a page that actually contains this album — the two
+-- diverging is exactly the bug #367 had to fix in ListAlbumsByGenre.
+SELECT DISTINCT trim(g.genre) AS genre
+FROM tracks
+JOIN LATERAL regexp_split_to_table(coalesce(tracks.genre, ''), '[;,]') AS g(genre) ON true
+WHERE tracks.album_id = $1 AND trim(g.genre) <> ''
+ORDER BY trim(g.genre);
+
+-- name: ListGenresForArtist :many
+-- Same, across everything by one artist. Alphabetical rather than by count:
+-- an artist's genre set is small, and a stable order reads better than a
+-- frequency ranking nobody asked about.
+SELECT DISTINCT trim(g.genre) AS genre
+FROM tracks
+JOIN LATERAL regexp_split_to_table(coalesce(tracks.genre, ''), '[;,]') AS g(genre) ON true
+WHERE tracks.artist_id = $1 AND trim(g.genre) <> ''
+ORDER BY trim(g.genre);