Add moveCookieToCategory mutation

Moving a cookie between categories previously required two sequential
updateCookieCategory mutations, which was not atomic and could leave
data in an inconsistent state if the second call failed. This adds a
dedicated moveCookieToCategory mutation that performs both updates in
a single transaction.

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/cookiebanner/errors.go

@@ -28,4 +28,6 @@ var (
 	ErrCannotDeleteSystemCategory = errors.New("cannot delete system cookie category")
 	ErrOriginAlreadyInUse         = errors.New("origin is already used by another active cookie banner")
 	ErrConsentNotFound            = errors.New("consent record not found")
+	ErrCookieNotFound             = errors.New("cookie not found in source category")
+	ErrCategoriesBannerMismatch   = errors.New("source and target categories belong to different banners")
 )