Clamp daemon push-sweep throttle to grace window (#83)

The onSaved retention sweep is meant to archive an aged-out previous
generation as soon as a new capture in the same page family arrives.
The fixed 5s push-sweep throttle added in #73 could swallow that sweep
when a second push lands within 5s of the first, leaving the old
generation in the listing past its grace window until the next 60m
timer sweep. This regressed daemon/scripts/e2e-smoke.mjs (onSaved
branch), which reproduced on main with retention.js unchanged.

Bound the throttle by the grace window (throttle <= grace): within the
grace window a rapid push cannot archive its predecessor anyway (it is
grace-protected), so throttling is free; once grace has elapsed the
predecessor becomes archivable and the throttle must not delay it. The
default grace (30m) far exceeds 5s, so production keeps the 5s throttle
unchanged; only an extremely short grace clamps it down.

Verified: daemon npm test 123/123, e2e-smoke green x3 (stable).

Author: buddypia

daemon/src/index.js

@@ -64,7 +64,13 @@ function sweep(dir) {
 // 度に inbox 全体を同期スキャンし直す必要はなく、次の定期 timer sweep でも同じ stale entry を
 // 安全に退避できる。起動時 sweep / 定期 timer sweep / downloadsDir 採用時の sweep は対象外
 // （頻度が低く意図的なタイミングのため、スロットルせず常に実行する）。
-const PUSH_SWEEP_THROTTLE_MS = 5000;
+//
+// 不変条件 throttle ≤ grace: grace 窓内では rapid push の旧世代はどのみち保護されて archive されない
+// ので throttle は無害だが、grace を「抜けた直後」の push で旧世代を即退避するのが onSaved sweep の
+// 役割であり、throttle がそれを食ってはならない（grace より長い throttle は、grace 経過後に archive
+// 可能になった旧世代の即時退避を次の定期 sweep まで遅らせてしまう）。既定 grace(30分) ≫ 5秒なので
+// 通常はそのまま 5秒。grace を極端に短く設定した場合のみ grace に切り詰める。
+const PUSH_SWEEP_THROTTLE_MS = Math.min(5000, retentionPolicy.graceWindowMs);
 let lastPushSweepAt = 0;
 function sweepAfterPush(dir) {
   const now = Date.now();