Swapping providers

The seam is the module boundary: the rest of the app only knows BillingService.getEntitlement(userId) and the two redirect endpoints. To use another provider, keep billing_subscriptions + the entitlement shape and replace the Stripe specifics:

1. Checkout/portal session creation in billing.service.ts → your provider's hosted equivalents.
2. billing-webhook.service.ts and billing-reversal.service.ts → your provider's webhook verification and event names. Keep the guarded-upsert pattern: every provider redelivers and reorders webhooks.
3. billing.config.ts env surface + the boot validation.

Mind the merchant-of-record difference: with MoR providers (Paddle, Lemon Squeezy) the provider is the seller and handles VAT; with Stripe you are the merchant (Stripe Tax calculates, you register/file).