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

CODEX_STATUS.md

@@ -182,7 +182,7 @@
 
 ## Полезные файлы
 - `AGENTS.md`
-- `docs/plan/mvp-system-plan.md`
+- `docs/plan/system-plan.md`
 - `docs/architecture/system-overview.md`
 - `docs/ops/deployment.md`
 - `docs/ops/provider-key-pool.md`