More accurate billing windows + refund handling

Usage now tracks against your actual billing period instead of the calendar month, so mid-month signups get exactly one full allowance per cycle. Full refunds correctly end access on the refund date.

• ›Token + USD allowance windows now align with your subscription period (no more two partial windows for mid-month signups)
• ›Full Stripe refunds now end access immediately and accurately
• ›Out-of-order billing events from Stripe are ignored if they're older than what we already processed