[Bug] Second user registration relies on implicit db.commit(), causing raw 500 on conflict

[jigangz]

Pre-checks

• I have searched existing issues and this is not a duplicate.

Deployment Method

Source code

Steps to Reproduce

1. Complete initial setup (first admin user created successfully)
2. Register a second user at /api/auth/register
3. If any database constraint is violated (e.g. duplicate email), the response is a raw 500 error instead of a proper JSON error

Expected vs Actual Behavior

Expected: A 409 Conflict JSON response with a clear error message.

Actual: A raw 500 Internal Server Error with no JSON body. The frontend shows a generic "Request failed" message.

Root Cause

In backend/app/api/auth.py, db.commit() is only called explicitly in the first-user (admin) registration path. For subsequent users, the commit is deferred to the get_db() dependency cleanup. If that deferred commit hits an IntegrityError, FastAPI cannot catch it and returns a raw 500.

Logs / Screenshots

No response