chore: migrate branding to backstagephp.com

Update author emails, seeders, mail tests, LICENSE and SECURITY docs
from ux.nl/vormkracht10.nl to backstagephp.com across packages.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mhortulanus

composer.json

@@ -9,6 +9,11 @@
     "homepage": "https://github.com/backstage/shop-support",
     "license": "MIT",
     "authors": [
+        {
+            "name": "Mark van Eijk",
+            "email": "mark@backstagephp.com",
+            "role": "Owner & Developer"
+        },
         {
             "name": "Manoj Hortulanus",
             "email": "manoj@backstagephp.com",