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/thirdparty/match.go

@@ -220,7 +220,7 @@ func LinkToCommon(
 // CreateFromCommon inserts a new org ThirdParty seeded from the catalog
 // row (name, category, addresses, URLs, certifications, …). The new row
 // has common_third_party_id pointed at commonParty, an empty Countries
-// list, ShowOnTrustCenter false, and FirstLevel true — the caller has
+// list, ShowOnTrustCenter false, and Level 1 — the caller has
 // already confirmed the vendor is actively present on the
 // organization's cookie banner, which makes it a first-level third
 // party by definition.
@@ -259,7 +259,7 @@ func CreateFromCommon(
 		SecurityPageURL:               commonParty.SecurityPageURL,
 		TrustPageURL:                  commonParty.TrustPageURL,
 		ShowOnTrustCenter:             false,
-		FirstLevel:                    true,
+		Level:                         1,
 		CreatedAt:                     now,
 		UpdatedAt:                     now,
 	}