test: add backend test data factories

[pntech20]

Summary

• add a typed backend test data factory for payment requests, meters, users, rate-limit config, and test env setup
• reuse the shared env helper from Jest setup so backend suites get one consistent test configuration
• sync the backend lockfile with the package manifest so installs include the declared mail dependencies

Checks

• npx jest src/test/testData.test.ts --runInBand --coverage=false
• git diff --check

Closes #158