Keep Review period keys in the scheduling time zone #78

Reproduction

1. Build a Review period with an explicit Asia/Bangkok calendar for 2026-08-01 through 2026-08-07.
2. Format its idempotency key while the process default time zone is UTC.
3. The current formatter emits 2026-07-31--2026-08-07-7d instead of 2026-08-01--2026-08-07-7d.

Risk

The same scheduled Review can acquire a different cursor key after a time-zone change. Eye may repeat a completed Review or treat the actual latest due period as unfinished.

Expected behavior

A Review period key is formatted in the same calendar/time zone that selected the period. Host locale and CI runner time zone must not alter it.

Acceptance criteria

• periodKey accepts or preserves the scheduling calendar/time zone.
• Daily and seven-day keys remain stable when the process default time zone differs.
• Bangkok and UTC regression fixtures prove the calendar is explicit.
• Existing stored key format remains unchanged.