#71 subtask - Domain: `users` + `project-users`

[kaseywright]

Files (4):

• src/domains/users/users.handlers.ts — 10 error return sites
• src/domains/users/users.route.ts — dispatch sites
• src/domains/projects/project-users/project-users.handlers.ts — 7 error return sites
• src/domains/projects/project-users/project-users.route.ts — 4 dispatch sites

Why grouped: project-users validates against the users table. Both files use handleUniqueConstraintError. Closely related user-management logic.

[LijuJacob08]

Review current code (ET - 2 hrs)
Update existing files to accommodate the changes (ET - 2 hrs)
Run checks to ensure no errors (ET - 2 hr)
Code review fixes (ET - 1.5 hr)
QA review fixes (ET - 1.5 hr)