feat(api): GET /api/artists/{id} with nested albums

internal/api/library.go

@@ -91,9 +91,47 @@ func (h *handlers) handleGetAlbum(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, detail)
 }
 
-// handleGetArtist is implemented in Task 8.
+// handleGetArtist implements GET /api/artists/{id}. Returns artist + albums;
+// each album carries its own track_count (one count query per album, same
+// pattern as subsonic). At M1 sizes (albums-per-artist << 100) this is fine.
 func (h *handlers) handleGetArtist(w http.ResponseWriter, r *http.Request) {
-	writeErr(w, http.StatusNotImplemented, "not_implemented", "stub")
+	id, ok := parseUUID(chi.URLParam(r, "id"))
+	if !ok {
+		writeErr(w, http.StatusBadRequest, "bad_request", "invalid artist id")
+		return
+	}
+	q := dbq.New(h.pool)
+	artist, err := q.GetArtistByID(r.Context(), id)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			writeErr(w, http.StatusNotFound, "not_found", "artist not found")
+			return
+		}
+		h.logger.Error("api: get artist failed", "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
+		return
+	}
+	albums, err := q.ListAlbumsByArtist(r.Context(), id)
+	if err != nil {
+		h.logger.Error("api: list albums by artist failed", "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
+		return
+	}
+	refs := make([]AlbumRef, 0, len(albums))
+	for _, a := range albums {
+		count, cerr := q.CountTracksByAlbum(r.Context(), a.ID)
+		if cerr != nil {
+			h.logger.Error("api: count tracks failed", "err", cerr)
+			writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
+			return
+		}
+		refs = append(refs, albumRefFrom(a, artist.Name, int(count), 0))
+	}
+	detail := ArtistDetail{
+		ArtistRef: artistRefFrom(artist, len(albums)),
+		Albums:    refs,
+	}
+	writeJSON(w, http.StatusOK, detail)
 }
 
 // handleListArtists is implemented in Task 9.