Replaces the implicit Account::booted() observer with an explicit
Account::forgetPlanFeatureCache() method called from each plan_id
mutation site (StripeEventListener x3, BillingController x2). Self-hosted
installs naturally never reach any of these callsites — Stripe webhooks
do not fire and the billing controllers redirect to /calendar before any
plan mutation happens — so the Pennant flush is now guaranteed to be a
cloud-only operation.
Adds integration coverage proving the full chain webhook -> plan_id
update -> Pennant flush -> next Feature::value resolves against the new
plan limit.
