Allow customers to modify subscriptions before auto-renewal occurs
## Problem
Currently, customers can only make subscription changes (seats, tier, add-ons) at the time of renewal - they cannot modify their subscription in advance and have those changes take effect on the renewal date. While auto-renewal customers can still renew manually to make changes, this defeats the purpose of having auto-renewal enabled. This creates friction for customers who want to plan subscription changes in advance while maintaining the convenience of automated renewals.
## Proposal
Enable customers to pre-configure subscription modifications that will take effect on their auto-renewal date, starting 30 days (exact number of days is TBD) before renewal:
* Allow customers to make the same subscription changes available during manual renewal, but configure them ahead of time
* Use the same renewal UI but provide a "save changes without processing renewal" option instead of immediate renewal processing
* All changes and payment processing occur on the scheduled auto-renewal date
This allows customers to maintain the convenience of auto-renewal while having control over what their renewed subscription will look like.
epic