Pure deterministic monthly period generator for the four ranking
families. Anchor dates derived from prod capture (2026-06-09): id=1 is
each family's launch month in JST; id=N is anchor + N-1 months. Used
by /ranking/get_viewable_ranking_period_list to render the period
picker and by per-family leaderboard endpoints to echo the requested
period back.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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