Fix registration token subject

[yZangEren]

Summary

• Generate the registration user id once and reuse it for the returned user and JWT sub
• Prevent mismatches when timestamp generation crosses a millisecond boundary during registration
• Add a focused regression test that forces different timestamp values

Tests

• node --test src\tests\auth.test.js src\tests\health.test.js
• git diff --check

Closes #4203
Refs #743

/claim #743

Demo

• Short demo video: https://raw.githubusercontent.com/yZangEren/bug-bounty/fix-register-token-subject/demos/register-token-subject-demo.mp4
• Fresh validation: node --test src\tests\auth.test.js src\tests\health.test.js passed.