Inviting an existing user as an organisation member causes an error

[Joe-Heffer-Shef]

Nicola report that this is error message that occurs:

An active user is using this e-mail address

[Joe-Heffer-Shef]

When an organisation admin invites a user who already has an account in SORT, the system throws an IntegrityError because:

1. ManagerSignupForm.save() (home/forms/manager_signup.py:63-78) attempts to create a new User with the invited email
2. The User.email field has a unique=True constraint (home/models.py:41)
3. Even if the user creation succeeded, organisation_service.add_user_to_organisation() would fail due to the unique_together constraint on OrganisationMembership (home/models.py:10

  Admin sends invite → MyOrganisationInviteView
      ↓
  Creates invitations.models.Invitation (django-invitations package)
      ↓
  Email sent with invitation link
      ↓
  User clicks link → MyOrganisationAcceptInviteView
      ↓
  Redirects to SignupView with invitation key
      ↓
  User submits form → ManagerSignupForm.save()
      ↓
  ❌ FAILS: Tries to create new User (line 68)
      OR
  ❌ FAILS: Tries to create OrganisationMembership (line 71-76)