Remove the sync.Mutex and CreateUserAtomic (INSERT ON CONFLICT) in favor
of a single DB transaction in CreateFirstUser that atomically checks for
existing users and inserts. SQLite serializes write transactions, so this
is sufficient to prevent the race condition without application-level locking.
Add mutex and INSERT ON CONFLICT to CreateUser to prevent TOCTOU race
where concurrent requests could create multiple admin users.
Changes:
- Add sync.Mutex to auth.Service to serialize CreateUser calls
- Add models.CreateUserAtomic using INSERT ... ON CONFLICT(username) DO NOTHING
- Check RowsAffected to detect conflicts at the DB level (defense-in-depth)
- Add concurrent race condition test (10 goroutines, only 1 succeeds)
The existing UNIQUE constraint on users.username was already in place.
This fix adds the application-level protection (items 1 & 2 from #26).
