Documents a revision, not a rewrite. The snapshot-on-Due pattern,
dated prices and the asymmetric dry-run commands are sound and are
kept; what's wrong is three hardcoded assumptions in models.py:
annual-only periods, a global GRACE_DAYS, and grace measured from
period_end -- which gives a club ~410 days of unpaid use before
archive_overdue_clubs will touch it.
Proposes per-plan duration/lead/grace, grace measured from
period_start, a Tier -> Plan rename, and a club-facing unpaid-dues
warning with a countdown to archiving.
Flags the migration hazard prominently: re-deriving grace_until on
existing dues under the new rule puts the date in the past for every
open annual period, which would archive the whole paying customer
base on the next --commit run.
