Getsentry billing charge path — 5-function call chain for plan upgrades

Getsentry billing charge path — 5-function chain: API → switch_performance_plan() → change_plan() → create_invoice() → capture_charge() → _perform_charge(). Custom billing via stripe.PaymentIntent.create() + confirm, no Subscriptions. BasePlan.id = Stripe Price ID. Plan manager: getsentry.billing.plans.manager. SPT hooks at capture_charge() with shared_payment_granted_token. Cell-silo subscription writes via cell_request() PUT to /api/0/customers/{org_slug}/subscription/ (not direct ORM). Subscription.plan is CharField; use subscription.current_plan for BasePlan object.