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.

README.md

@@ -133,7 +133,7 @@ Authenticated users who have solved today's puzzle can submit a verse for schedu
 | --- | --- |
 | `POST /api/daily-verse` | Fetch (and cache) the verse for a given `YYYY-MM-DD` date. Also records a deduped `verse_served` event for the supplied `anonymousId` (started→finished funnel). |
 | `POST /api/events` | Fire-and-forget analytics sink. Allowlisted names: `verse_served`, `share`, `share_copy`, `first_visit`, `donate_click`, `discord_click`; `userId` resolved from the session. Always returns 204. |
-| `POST /api/cron/build-stats` | Cron-only (bearer `CRON_SECRET`). Builds yesterday's snapshot (`America/New_York`) and posts the digest to `DISCORD_ADMIN_WEBHOOK`. |
+| `POST /api/cron/build-stats` | Cron-only (bearer `CRON_SECRET`). Builds the most recent snapshot whose global data window has closed (`D+1 12:00Z`) and posts the digest to `DISCORD_ADMIN_WEBHOOK`. |
 | `POST /api/admin/rebuild-stats` | Admin-only. Rebuilds one snapshot: `{ date: 'YYYY-MM-DD' }`. |
 | `POST /api/submit-completion` | Submit a game result (`anonymousId`, `date`, `guessCount`, `guesses`); returns solve rank, guess rank, total solves, average guesses, ties, and percentile. Unique on `(anonymousId, date)`. |
 | `GET /api/streak?anonymousId=X&localDate=Y` | Current streak: walks backwards from the client's local date through completed dates, counting consecutive days. Single-day streaks are reported as 0 (minimum displayed streak is 2). |