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/metrics.go

@@ -69,22 +69,20 @@ func StoryViewTotals(ids []int64) map[int64]int {
 		}
 		out[id] = n
 	}
+	if err := rows.Err(); err != nil {
+		slog.Error("metrics: story view totals iteration failed", "err", err)
+	}
 	return out
 }
 
 // intInClause builds a "?, ?, …" placeholder string and matching args slice for
 // a SQL IN (…) over int64 ids.
 func intInClause(ids []int64) (string, []any) {
-	ph := make([]byte, 0, len(ids)*2)
 	args := make([]any, len(ids))
 	for i, id := range ids {
-		if i > 0 {
-			ph = append(ph, ',')
-		}
-		ph = append(ph, '?')
 		args[i] = id
 	}
-	return string(ph), args
+	return placeholders(len(ids)), args
 }
 
 // PathStat is one row of the per-page usage breakdown.