Fix reader read-aloud races and add rows.Err checks

- reader.js: guard read-aloud against the loading placeholder (bodyReady)
  and invalidate stale speechSynthesis callbacks with a generation token
- storage: check rows.Err() after iterating story-view/content-length reads
- metrics: reuse placeholders() instead of duplicating the IN-clause builder

internal/storage/queries.go

@@ -438,6 +438,9 @@ func StoryContentLengths(ids []int64) map[int64]int {
 		}
 		out[id] = int(n)
 	}
+	if err := rows.Err(); err != nil {
+		slog.Error("story content lengths iteration failed", "err", err)
+	}
 	return out
 }