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/coredata/third_party_vetting.go

@@ -32,6 +32,7 @@ func (v *ThirdParty) LoadNextPendingVettingForUpdateSkipLocked(
 SELECT
     id,
     organization_id,
+    parent_third_party_id,
     common_third_party_id,
     name,
     description,
@@ -53,7 +54,7 @@ SELECT
     security_page_url,
     trust_page_url,
     show_on_trust_center,
-    first_level,
+    level,
     vetting_status,
     vetting_website_url,
     vetting_procedure,