Closes#2
## Summary
- make `markInvoicePaid` idempotent for already-paid invoices and reject invalid terminal transitions
- add admin actor metadata and audit-log writes for `mark-paid`, including replayed no-op calls
- add focused DB tests for first activation, replay safety, and invalid transition handling
- document the current payment system, including invoice creation, manual activation, quota reset, and current limitations
## 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
Co-authored-by: sirily <sirily@git.shararam.party>
Reviewed-on: #18
