Scope sub-third-parties per parent

Replace the many-to-many junction table with a direct
parent_third_party_id foreign key on third_parties. Each
sub-third-party now belongs to exactly one parent, making
duplicates across parents independent entities.

Replace the firstLevel boolean with an integer level field
(1 = direct, 2+ = parent level + 1) to support arbitrary
nesting depth.

Remove the createThirdPartyThirdPartyMapping and
deleteThirdPartyThirdPartyMapping mutations, the CLI
link/unlink commands, and the corresponding MCP tools.
Creating a child third party now just requires passing
parentThirdPartyId on the existing createThirdParty mutation.

The frontend walks the parentThirdParty chain to build
display names like "Name (Ancestor1/Ancestor2)" and shows
clickable ancestor links on the detail page.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/iam/organization_service.go

@@ -767,6 +767,7 @@ func (s *OrganizationService) CreateOrganization(
 				TermsOfServiceURL:    &proboThirdParty.TermsOfServiceURL,
 				SubprocessorsListURL: &proboThirdParty.SubprocessorsListURL,
 				ShowOnTrustCenter:    false,
+				Level:                1,
 				CreatedAt:            now,
 				UpdatedAt:            now,
 			}