fix(ci): validate SONAR_HOST_URL before running scanner

Fail early with a clear message if the variable is missing or lacks
an http(s) scheme.

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

.gitea/workflows/sonarqube.yml

@@ -46,6 +46,10 @@ jobs:
           SONAR_HOST_URL: ${{ vars.SONAR_HOST_URL }}
           SONAR_TOKEN: ${{ secrets.SONAR_TOKEN }}
         run: |
+          if [ -z "$SONAR_HOST_URL" ] || ! echo "$SONAR_HOST_URL" | grep -qE '^https?://'; then
+            echo "::error::SONAR_HOST_URL is missing or invalid (got: '$SONAR_HOST_URL'). Set it as a repo variable with full URL (e.g. https://sonar.example.com)."
+            exit 1
+          fi
           sonar-scanner \
             -Dsonar.host.url="$SONAR_HOST_URL" \
             -Dsonar.login="$SONAR_TOKEN" \