Tool shall use internal email template

[mxmehl]

Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.12.9/x64/bin/auth-user-mgr", line 8, in <module>
    sys.exit(cli())
             ^^^^^
  File "/opt/hostedtoolcache/Python/3.12.9/x64/lib/python3.12/site-packages/auth_user_mgr/main.py", line 2[52](https://github.com/OpenRailAssociation/openrail-users/pull/2/checks#step:7:53), in cli
    if user_check_existence(api=api, user=user, mail=mail):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.12.9/x64/lib/python3.12/site-packages/auth_user_mgr/main.py", line 139, in user_check_existence
    mail.send_email(recipient=user.email, link=invitation_url)
  File "/opt/hostedtoolcache/Python/3.12.9/x64/lib/python3.12/site-packages/auth_user_mgr/_email.py", line 75, in send_email
    template_str = self.read_template(self.template)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.12.9/x64/lib/python3.12/site-packages/auth_user_mgr/_email.py", line 67, in read_template
    with open(file_path, "r", encoding="utf-8") as file:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: 'templates/invitation.html.j2'