fix: make invoice payment activation idempotent (#18)

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

packages/db/package.json

@@ -21,7 +21,9 @@
     "db:push": "prisma db push",
     "generate": "prisma generate",
     "migrate:deploy": "prisma migrate deploy",
-    "format": "prisma format"
+    "format": "prisma format",
+    "pretest": "pnpm build",
+    "test": "node --test dist/**/*.test.js"
   },
   "dependencies": {
     "@nproxy/domain": "workspace:*",