fix(stats): finalize snapshots on the verse-day window

A verse date is a player's local calendar day, spanning D-1 10:00Z
(UTC+14) through D+1 12:00Z (UTC-12). The old UTC end-of-day cutoff both
dropped late-night finishes in negative timezones and could persist a
partial day.

- Scope snapshot loads by the verse date column; use completedBefore only
  to pin an in-progress point-in-time build.
- Gate persistence on isDayClosed (D+1 12:00Z); open days are computed
  live and never written.
- Build the cron snapshot for latestClosedDate instead of EST yesterday,
  and read the same date for the community page.
- Add all-time best single day to snapshot totals.
- Bump PAYLOAD_SCHEMA_VERSION to 4 and extend snapshot/window tests.

tests/stats-digest.test.ts

@@ -40,7 +40,7 @@ const payload: SnapshotPayload = {
 	},
 	habit: { streakSurvival: [], streakLengths: [], calendar: [], weekdayHour: [] },
 	growth: { lifecycleFunnel: [], signups: [] },
-	totals: { completions: 31775, players: 6589, yesterdayCompletions: 102, yesterdayAvgGuesses: 4.1 },
+	totals: { completions: 31775, players: 6589, yesterdayCompletions: 102, yesterdayAvgGuesses: 4.1, bestDay: null },
 	signals: [
 		{
 			id: 'd1_return',