feat(api): GET /api/artists with alpha/newest sort and paging

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/library.go

@@ -134,9 +134,64 @@ func (h *handlers) handleGetArtist(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, detail)
 }
 
-// handleListArtists is implemented in Task 9.
+// handleListArtists implements GET /api/artists with two sort modes
+// (alpha|newest) and enveloped pagination. album_count per artist is
+// computed via an N+1 — acceptable at limit=200.
 func (h *handlers) handleListArtists(w http.ResponseWriter, r *http.Request) {
-	writeErr(w, http.StatusNotImplemented, "not_implemented", "stub")
+	sort := r.URL.Query().Get("sort")
+	if sort == "" {
+		sort = "alpha"
+	}
+	if sort != "alpha" && sort != "newest" {
+		writeErr(w, http.StatusBadRequest, "bad_request", "sort must be alpha or newest")
+		return
+	}
+	limit, offset, err := parsePaging(r.URL.Query())
+	if err != nil {
+		writeErr(w, http.StatusBadRequest, "bad_request", err.Error())
+		return
+	}
+
+	q := dbq.New(h.pool)
+	var artists []dbq.Artist
+	switch sort {
+	case "newest":
+		artists, err = q.ListArtistsNewest(r.Context(), dbq.ListArtistsNewestParams{
+			Limit: int32(limit), Offset: int32(offset),
+		})
+	default: // alpha
+		artists, err = q.ListArtistsAlpha(r.Context(), dbq.ListArtistsAlphaParams{
+			Limit: int32(limit), Offset: int32(offset),
+		})
+	}
+	if err != nil {
+		h.logger.Error("api: list artists failed", "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
+		return
+	}
+	total, err := q.CountArtists(r.Context())
+	if err != nil {
+		h.logger.Error("api: count artists failed", "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "count failed")
+		return
+	}
+
+	items := make([]ArtistRef, 0, len(artists))
+	for _, a := range artists {
+		albums, aerr := q.ListAlbumsByArtist(r.Context(), a.ID)
+		if aerr != nil {
+			h.logger.Error("api: list albums for artist failed", "err", aerr)
+			writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
+			return
+		}
+		items = append(items, artistRefFrom(a, len(albums)))
+	}
+	writeJSON(w, http.StatusOK, Page[ArtistRef]{
+		Items:  items,
+		Total:  int(total),
+		Limit:  limit,
+		Offset: offset,
+	})
 }
 
 // handleSearch is implemented in Task 10 (file: search.go).