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/cookiebanner/tracker_mapping_worker_test.go

@@ -255,7 +255,7 @@ func TestPromoteThirdParty_FallbackCreate(t *testing.T) {
 	require.NotNil(t, reloaded.CommonThirdPartyID)
 	assert.Equal(t, fx.commonThirdPartyID, *reloaded.CommonThirdPartyID)
 	assert.Equal(t, coredata.ThirdPartyCategoryAnalytics, reloaded.Category)
-	assert.True(t, reloaded.FirstLevel)
+	assert.Equal(t, 1, reloaded.Level)
 	assert.False(t, reloaded.ShowOnTrustCenter)
 }