FluentCart subscriptions and renewals

Model both who creates renewal invoices and who charges them. Do not treat every recurring plan as a remote provider subscription.

Read subscription-lifecycle.md before adding access, billing, gateway, reminder, or dunning behavior.

Distinguish billing modes

The store setting chooses gateway_managed or store_managed for new subscriptions and stamps management_mode into the subscription config. Changing the store setting later must not silently change existing contracts.