Improved Skip rules

Skip Limits now supports a calendar-year mode, so you can enforce policies like "one skip per calendar year" without relying on rolling-day math — counts reset on Jan 1 in your shop's timezone. Skip enforcement is also now wired into the admin subscription detail view, matching the customer portal: the Skip button is disabled with a clear tooltip when a subscription has hit its limit. Reschedules that move billing past the upcoming batch date are still allowed for staff flexibility, but skip caps continue to apply once a subscription reaches the limit.