Dodo Payments Gateway Support

Muse Billing now supports Dodo Payments for subscriptions and one-time checkout, with setup docs, env templates, and a lifetime entitlements home surface.

This release adds Dodo Payments as a first-class Muse Billing gateway, so teams can launch checkout, webhooks, and customer portal flows alongside Creem and Stripe.

Dodo Payments Integration

Added the muse_dodo gateway with checkout session launch, webhook verification, contract sync, and customer portal links.
Extended product catalog and environment templates with Dodo API keys, webhook secrets, environment mode, and lifetime/monthly/yearly product ID variables.
Registered the gateway in Muse Billing orchestration with unit and API coverage for gateway behavior, mappers, and access resolution.