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

README.md

@@ -1,12 +1,12 @@
 # nproxy
 
-Planning scaffold for a crypto-subscription image gateway.
+Product codebase for a crypto-subscription image gateway.
 
-The repository is intentionally light on runtime code. Its current purpose is to store:
-- the agreed MVP plan;
-- the repository layout for future implementation;
+The repository contains:
+- runtime applications and shared packages;
+- the agreed system plan and architecture documents;
 - operational notes for deployment, Telegram pairing, and provider key rotation;
-- directory-scoped instructions so future Codex runs can implement against the same decisions.
+- directory-scoped instructions so Codex runs implement against the same decisions.
 
 ## Chosen baseline
 - Product: B2C website
@@ -25,7 +25,7 @@ The repository is intentionally light on runtime code. Its current purpose is to
 - `scripts/` operational helpers
 
 ## Read first
-- `docs/plan/mvp-system-plan.md`
+- `docs/plan/system-plan.md`
 - `docs/architecture/system-overview.md`
 - `docs/ops/deployment.md`
 - `CONTRIBUTING.md`