fix: derive JAVA_HOME for sonar-scanner in CI workflow

sonar-scanner 6.x requires JAVA_HOME or java on PATH. The build
container has Java installed but doesn't export JAVA_HOME, so
derive it from the java binary location.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/sonarqube.yml

@@ -72,6 +72,10 @@ jobs:
 
       - name: SonarQube Analysis
         run: |
+          # Derive JAVA_HOME from the java binary on PATH (set by build image)
+          if [ -z "$JAVA_HOME" ]; then
+            export JAVA_HOME=$(dirname $(dirname $(readlink -f $(which java))))
+          fi
           sonar-scanner \
             -Dsonar.host.url="$SONAR_HOST_URL" \
             -Dsonar.token="$SONAR_TOKEN" \