[StripeCardholder] Clean up Stripe update code paths

StripeCardholder data is updated in a couple ways. They potentially conflict or update Stripe twice.

- StripeCardholder before_validation
- This probably should be an after_save
- StripeCardholderService::Update
- UserController#update

Start by reading the three named update paths: StripeCardholder#before_validation, StripeCardholderService::Update, and UserController#update. Trace how each updates Stripe and identify where their behavior conflicts or duplicates work. Done means the StripeCardholder update flow has one consistent path without conflicting or repeated Stripe updates.

Written by the indexing model from the issue text.