Support independent billing and credit-grant cadences for subscriptions

Merchants want to bill a subscription on one cadence (e.g., annually) while granting credits on a shorter, separate cadence (e.g., monthly) — so customers can't front-load and consume the full period's credit allowance immediately after paying.

Currently, credit grants are tied to the subscription's billing cycle only, so merchants have to self-manage interim grants via the Credit Ledger API (creditEntitlements.balances.createLedgerEntry) with manual expires_at windows as a workaround, while still wanting Dodo to remain the source of truth for balances, expiry, rollover, and overage.

Native support for a configurable grant cadence independent of the billing cadence would remove this workaround.