fix(auth): require organization invitations for wallet registration

[diiabblo]

Summary

• replace automatic wallet registration with an invitation-only registration endpoint
• issue single-use, hashed organization invitations for employers
• assign new employee accounts to the invitation organization atomically before issuing a session

Testing

• NODE_OPTIONS=--experimental-vm-modules npm test -- --runInBand src/controllers/__tests__/authController.test.ts
• npm run build is blocked by pre-existing syntax errors in src/services/tenantConfigService.ts (lines 207-221)

Closes #541

[Wilfred007]

Thank you @diiabblo