test: make Windows maintenance concurrency deterministic

[khaira777]

Summary

• replace the fixed 20 ms maintenance delay with an explicit promise gate
• keep maintenance active until the concurrent route assertion completes
• preserve the 503-before-auth regression coverage without changing production code

Validation

• npm run test:database-tools-api
• npm test
• npm run build
• npm run lint
• focused maintenance test repeated 20 times

Fixes the Windows-only 200 !== 503 flake in database-maintenance-lock.test.ts.