feat(api): add GET /api/artists/{id}/tracks handler

Wires ListArtistTracksForUser (Task 3 SQL) to a new handler that returns
all quarantine-filtered tracks for an artist as a flat TrackRef list.
404 when the artist doesn't exist. Route registered in Mount().

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

internal/api/library.go

@@ -143,6 +143,46 @@ func (h *handlers) handleGetArtist(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, detail)
 }
 
+// handleGetArtistTracks implements GET /api/artists/{id}/tracks. Returns
+// every track the artist has across albums (per-user-quarantine filtered)
+// as a flat list. Used by ArtistCard's play affordance, which shuffles
+// client-side. 404 when the artist doesn't exist.
+func (h *handlers) handleGetArtistTracks(w http.ResponseWriter, r *http.Request) {
+	id, ok := parseUUID(chi.URLParam(r, "id"))
+	if !ok {
+		writeErr(w, http.StatusBadRequest, "bad_request", "invalid artist id")
+		return
+	}
+	user, ok := auth.UserFromContext(r.Context())
+	if !ok {
+		writeErr(w, http.StatusUnauthorized, "unauthorized", "authentication required")
+		return
+	}
+	q := dbq.New(h.pool)
+	if _, err := q.GetArtistByID(r.Context(), id); err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			writeErr(w, http.StatusNotFound, "not_found", "artist not found")
+			return
+		}
+		h.logger.Error("api: get artist for tracks", "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
+		return
+	}
+	rows, err := q.ListArtistTracksForUser(r.Context(), dbq.ListArtistTracksForUserParams{
+		ArtistID: id, UserID: user.ID,
+	})
+	if err != nil {
+		h.logger.Error("api: list artist tracks", "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
+		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)
+}
+
 // 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.