Add Gitea Actions workflow to run PHPUnit on pull requests

Runs the backend test suite against PHP 8.3 for every PR targeting
main, so branch protection can require it to pass before merging.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -0,0 +1,21 @@
+name: CI
+
+on:
+  pull_request:
+    branches: [main]
+
+jobs:
+  php-tests:
+    runs-on: ubuntu-latest
+    container: php:8.3-cli-alpine
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Composer
+        run: curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
+
+      - name: Install dependencies
+        run: composer install --no-progress --prefer-dist
+
+      - name: Run tests
+        run: vendor/bin/phpunit