feat(subplat): add free access program to billing-and-subscriptions API
Because: * Relying parties fetch current subscriptions via GET v1/billing-and-subscriptions, but Free Access Program grants were not represented. This commit: * Adds a `free_access` subscription type sourcing product identity from the related offering. * Filters grants by the projection's per-client capabilities. * Gates the code behind a FreeAccessProgramConfig feature flag (default off). * Add webhook handler to payments-api to clear cache Closes #PAY-3790