feat(server): similar-artists + per-user artist top-tracks endpoints

GET /api/artists/{id}/similar — in-library artists ranked by similarity
score (deduped across sources), ArtistRef list with cover + album count.
GET /api/artists/{id}/top-tracks — current user's most-played tracks for
the artist (skips excluded, quarantine filtered).

internal/api/api.go

@@ -86,6 +86,8 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 			authed.Get("/artists", h.handleListArtists)
 			authed.Get("/artists/{id}", h.handleGetArtist)
 			authed.Get("/artists/{id}/tracks", h.handleGetArtistTracks)
+			authed.Get("/artists/{id}/similar", h.handleGetSimilarArtists)
+			authed.Get("/artists/{id}/top-tracks", h.handleGetArtistTopTracks)
 			authed.Get("/albums/{id}", h.handleGetAlbum)
 			authed.Get("/albums/{id}/cover", h.handleGetCover)
 			authed.Get("/library/shuffle", h.handleLibraryShuffle)

internal/api/library.go

@@ -159,6 +159,85 @@ func (h *handlers) handleGetArtistTracks(w http.ResponseWriter, r *http.Request)
 	writeJSON(w, http.StatusOK, out)
 }
 
+// similarArtistsDefaultLimit caps the artist-detail "similar artists" strip;
+// artistTopTracksDefaultLimit caps the per-user "top tracks" panel.
+const (
+	similarArtistsDefaultLimit  = 12
+	artistTopTracksDefaultLimit = 5
+)
+
+// handleGetSimilarArtists implements GET /api/artists/{id}/similar. Returns
+// in-library artists similar to {id} (ranked by similarity score) as a flat
+// ArtistRef list with cover + album count. Empty when the similarity ingest
+// has no matches yet; 404 when the artist doesn't exist.
+func (h *handlers) handleGetSimilarArtists(w http.ResponseWriter, r *http.Request) {
+	id, ok := requireURLUUID(w, r, "id")
+	if !ok {
+		return
+	}
+	q := dbq.New(h.pool)
+	if _, err := q.GetArtistByID(r.Context(), id); err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			writeErr(w, apierror.NotFound("artist"))
+			return
+		}
+		h.logger.Error("api: get artist for similar", "err", err)
+		writeErr(w, apierror.InternalMsg("lookup failed", err))
+		return
+	}
+	rows, err := q.ListSimilarArtistsForArtist(r.Context(), dbq.ListSimilarArtistsForArtistParams{
+		SeedArtistID: id, ResultLimit: similarArtistsDefaultLimit,
+	})
+	if err != nil {
+		h.logger.Error("api: list similar artists", "err", err)
+		writeErr(w, apierror.InternalMsg("lookup failed", err))
+		return
+	}
+	out := make([]ArtistRef, 0, len(rows))
+	for _, row := range rows {
+		out = append(out, artistRefFromCovered(row.Artist, int(row.AlbumCount), row.CoverAlbumID))
+	}
+	writeJSON(w, http.StatusOK, out)
+}
+
+// handleGetArtistTopTracks implements GET /api/artists/{id}/top-tracks. Returns
+// the current user's most-played tracks for {id} (skips excluded, quarantine
+// filtered). Empty when the user hasn't played this artist; 404 when the
+// artist doesn't exist.
+func (h *handlers) handleGetArtistTopTracks(w http.ResponseWriter, r *http.Request) {
+	id, ok := requireURLUUID(w, r, "id")
+	if !ok {
+		return
+	}
+	user, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	q := dbq.New(h.pool)
+	if _, err := q.GetArtistByID(r.Context(), id); err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			writeErr(w, apierror.NotFound("artist"))
+			return
+		}
+		h.logger.Error("api: get artist for top tracks", "err", err)
+		writeErr(w, apierror.InternalMsg("lookup failed", err))
+		return
+	}
+	rows, err := q.ListMostPlayedTracksForArtist(r.Context(), dbq.ListMostPlayedTracksForArtistParams{
+		ArtistID: id, UserID: user.ID, ResultLimit: artistTopTracksDefaultLimit,
+	})
+	if err != nil {
+		h.logger.Error("api: list artist top tracks", "err", err)
+		writeErr(w, apierror.InternalMsg("lookup failed", err))
+		return
+	}
+	out := make([]TrackRef, 0, len(rows))
+	for _, row := range rows {
+		out = append(out, trackRefFrom(row.Track, row.AlbumTitle, row.ArtistName))
+	}
+	writeJSON(w, http.StatusOK, out)
+}
+
 // handleLibraryShuffle implements GET /api/library/shuffle?limit=N —
 // the online source for the client's always-present "Shuffle all"
 // (#427 S4). N random tracks across the whole library, per-user