feat: Use git submodule for private dependency

- Add wc-licensed-product-client as git submodule in lib/
- Change Composer repository from VCS to path type
- CI now checks out submodules recursively
- Remove COMPOSER_AUTH (no longer needed)

This solves the CI networking issue by fetching the
dependency via git submodule instead of Composer.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

composer.json

@@ -16,8 +16,8 @@
     },
     "repositories": [
         {
-            "type": "vcs",
-            "url": "https://src.bundespruefstelle.ch/magdev/wc-licensed-product-client.git"
+            "type": "path",
+            "url": "lib/wc-licensed-product-client"
         }
     ],
     "require": {