All JstPeriod callsites have moved to IGameCalendarService — delete the
static class and its tests. Updates the doc comments on Viewer.cs
(LastLoginBonusClaimedAt) and ViewerEventCounter.cs to reference the
new UTC / config-driven boundary instead of the old "JST 02:00".
GameConfigurationJsonb config-section snapshot test picks up "GameCalendar".
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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