ci: add PHPUnit pipeline (#65)

Neue Gitea-CI-Pipeline analog zur bestehenden lint.yml:
- Trigger auf push und pull_request
- Installiert PHP + Composer + Extensions (xml, mbstring)
- Fuehrt 'composer install' und 'vendor/bin/phpunit' aus
- 18 Tests / 31 Assertions gruen

.gitea/workflows/phpunit.yml

@@ -0,0 +1,25 @@
+name: PHPUnit
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  phpunit:
+    name: PHP Unit Tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install PHP & Composer
+        run: |
+          apt-get update -qq
+          apt-get install -y -qq php-cli php-xml php-mbstring composer > /dev/null 2>&1
+          php --version
+          composer --version
+
+      - name: Install dependencies
+        run: composer install --no-interaction --prefer-dist --no-progress
+
+      - name: Run PHPUnit
+        run: vendor/bin/phpunit