docs: choose BTCPay for payments

docs/plan/system-plan.md

@@ -8,7 +8,7 @@ The service hides provider-key failures behind a routed key pool. A user request
 ## Confirmed product decisions
 - One B2C website.
 - One monthly subscription plan.
-- Crypto checkout through a payment processor.
+- Crypto checkout through self-hosted `BTCPay Server`.
 - Manual renewal.
 - Text-to-image and image-to-image.
 - User-facing synchronous experience implemented with polling over background execution.
@@ -93,6 +93,7 @@ Single VPS with Docker Compose, expected services:
 - `bot`
 - `postgres`
 - `caddy` or `nginx`
+- `btcpay`
 - optional `minio` when object storage is self-hosted
 
 ## Optional extensions