feat: [1/n][role experiement] user model changes

[surbhi-posthog]

Problem

For experiment details please refer to : #26993

We previously did not have role in the user role in the user model. To expose role based changes in the UX, we need to add this to our User model

Changes

backend model changes

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

Does this work well for both Cloud and self-hosted?

n/a

How did you test this code?

In python debugger:
created a new user with and without a role and made sure we can read them correctly:
from posthog.models import User

# 1. Create user with role
user_with_role = User.objects.create_user(
    email="[email protected]",
    password="testpassword123",
    first_name="Test",
    role_at_organization="engineering"
)
print(f"User with role: {user_with_role.email}, Role: {user_with_role.role_at_organization}")

# 2. Create user without role
user_without_role = User.objects.create_user(
    email="[email protected]",
    password="testpassword123",
    first_name="Test"
)
print(f"User without role: {user_without_role.email}, Role: {user_without_role.role_at_organization}")