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/api/convert.go

@@ -183,3 +183,13 @@ func parsePaging(raw url.Values) (limit, offset int, err error) {
 	}
 	return limit, offset, nil
 }
+
+// nonNilStrings guarantees a JSON array rather than null. The clients iterate
+// these without a null check, matching how every other list field in this
+// package is emitted.
+func nonNilStrings(in []string) []string {
+	if in == nil {
+		return []string{}
+	}
+	return in
+}

internal/api/library.go

@@ -82,9 +82,17 @@ func (h *handlers) handleGetAlbum(w http.ResponseWriter, r *http.Request) {
 		refs = append(refs, ref)
 		durSec += ref.DurationSec
 	}
+	// Genre chips are a navigation nicety, so a failure here must not 404 an
+	// album that loaded fine. Log and ship the detail without them.
+	genres, err := q.ListGenresForAlbum(r.Context(), album.ID)
+	if err != nil {
+		h.logger.Warn("api: list album genres failed", "err", err, "album_id", uuidToString(album.ID))
+		genres = nil
+	}
 	detail := AlbumDetail{
 		AlbumRef: albumRefFrom(album, artistName, len(tracks), durSec),
 		Tracks:   refs,
+		Genres:   nonNilStrings(genres),
 	}
 	writeJSON(w, http.StatusOK, detail)
 }
@@ -114,9 +122,15 @@ func (h *handlers) handleGetArtist(w http.ResponseWriter, r *http.Request) {
 		// durationSec=0: not aggregated for nested album lists per spec data flow.
 		refs = append(refs, albumRefFrom(row.Album, artist.Name, int(row.TrackCount), 0))
 	}
+	genres, err := q.ListGenresForArtist(r.Context(), artist.ID)
+	if err != nil {
+		h.logger.Warn("api: list artist genres failed", "err", err, "artist_id", uuidToString(artist.ID))
+		genres = nil
+	}
 	detail := ArtistDetail{
 		ArtistRef: artistRefFrom(artist, len(rows)),
 		Albums:    refs,
+		Genres:    nonNilStrings(genres),
 	}
 	writeJSON(w, http.StatusOK, detail)
 }

internal/api/types.go

@@ -89,12 +89,19 @@ type TrackRef struct {
 type ArtistDetail struct {
 	ArtistRef
 	Albums []AlbumRef `json:"albums"`
+	// Genres carried by this artist's tracks, for quick-jump chips (#367).
+	// Always non-nil at JSON so the client can iterate without a null check.
+	Genres []string `json:"genres"`
 }
 
 // AlbumDetail is the response body of GET /api/albums/{id}.
 type AlbumDetail struct {
 	AlbumRef
 	Tracks []TrackRef `json:"tracks"`
+	// Genres carried by this album's tracks, for quick-jump chips (#367).
+	// Derived from the tracks rather than stored on the album, because genre
+	// lives on tracks and an album's tracks can disagree. Non-nil at JSON.
+	Genres []string `json:"genres"`
 }
 
 // SearchResponse is the body of GET /api/search. Each facet carries its own