* [PM-34813] fix system coupons regression
refactor customer setup class to split system coupons from discount coupons so that they can be applied systematically
* fix(billing): replace per-change IsStructural with changeset-level ChargeImmediately flag
* fix(billing): set seat quantity when upgrading from non-seat-based to seat-based plan
* chore: add CLAUDE.local.md and .worktrees to gitignore
* feat(billing): add Stripe interval and payment behavior constants and feature flag
* feat(billing): add OrganizationSubscriptionChangeSet model and unit tests
* refactor(billing): rename UpdateOrganizationSubscriptionCommand to BulkUpdateOrganizationSubscriptionsCommand
* feat(billing): add UpdateOrganizationSubscriptionCommand with tests
* feat(billing): use UpdateOrganizationSubscriptionCommand in BulkUpdateOrganizationSubscriptions behind feature flag
* feat(billing): use UpdateOrganizationSubscriptionCommand in SetUpSponsorshipCommand behind feature flag
* feat(billing): add UpgradeOrganizationPlanVNextCommand with tests and feature flag gate
* feat(billing): use UpdateOrganizationSubscriptionCommand in OrganizationService.AdjustSeatsAsync behind feature flag
* feat(billing): use UpdateOrganizationSubscriptionCommand in UpdateSecretsManagerSubscriptionCommand behind feature flag
* feat(billing): use UpdateOrganizationSubscriptionCommand in BillingHelpers.AdjustStorageAsync behind feature flag
* chore: run dotnet format
* fix(billing): missed optional owner in OrganizationBillingService.Finalize after merge
* refactor(billing): address PR feedback on UpdateOrganizationSubscription
