chore: remove MVP positioning and align plan defaults (#15)

## Summary
- remove MVP wording from repository docs and guidance
- rename the system plan document and update references to it
- align the default subscription plan code/name with product wording
- document hard subscription expiry with no grace period

## Verification
- docker build -f infra/docker/web.Dockerfile .
- docker build -f infra/docker/migrate.Dockerfile .

Co-authored-by: sirily <sirily@git.shararam.party>
Reviewed-on: #15

packages/db/README.md

@@ -4,7 +4,7 @@ Database package for `nproxy`.
 
 ## Implemented in this iteration
 - Prisma package scaffold
-- Initial Prisma schema for MVP persisted state
+- Current Prisma schema for persisted state
 - Shared schema path export for runtime tooling
 
 ## Current scope