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

infra/caddy/Caddyfile

@@ -1,4 +1,4 @@
-# Placeholder reverse-proxy config for the future Docker Compose deployment.
+# Placeholder reverse-proxy config for the Docker Compose deployment.
 # Replace example.com and upstream targets during implementation.
 
 example.com {