BUG: User team can be changed even when referenced tickets exist

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

## Problem
There is no database-level enforcement preventing a user's
team from being changed when tickets reference them as
created_by or assigned_to.

## Impact
Changing a user's team while they have tickets causes data
integrity issues — tickets end up referencing a user in a
different team than the ticket's team.

## Steps to Reproduce
1. Create a user in team A
2. Create a ticket with that user as created_by
3. Change the user's team to team B
4. Ticket now has created_by from a different team — no error thrown

## Expected Behavior
Database should raise an error and block the team change.

## Fix
Add prevent_user_team_change_if_tickets_exist() trigger
on users table.

### Record

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- [x] I want to work on this issue