Refs #9
## Summary
- add a worker-side renewal invoice sweep that creates one invoice 72 hours before subscription expiry
- expire elapsed pending invoices automatically and email users when an automatic renewal invoice is created
- stop auto-recreating invoices for the same paid cycle once any invoice already exists for that cycle
- document the current renewal-invoice and pending-invoice expiry behavior
## Testing
- built `infra/docker/web.Dockerfile`
- ran `pnpm --filter @nproxy/db test` inside the built container
- verified `@nproxy/db build` and `@nproxy/web build` during the image build
- built `infra/docker/worker.Dockerfile`
Co-authored-by: sirily <sirily@git.shararam.party>
Reviewed-on: #20
