fix: add background task to onboarding order sync + migrate to GitLab

Onboarding fixes:
- Add missing background task trigger for order sync (step 4)
- Import process_historical_import task in onboarding API

GitLab migration:
- Update audit rules to support both GitHub and GitLab paths
- Add .gitlab-ci.yml with lint, test, security, build stages
- Add merge request template (.gitlab/merge_request_templates/default.md)
- Update validate_audit.py to check for GitLab equivalents

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.audit-rules/documentation.yaml

@@ -94,6 +94,7 @@ rules:
       paths:
         - "SECURITY.md"
         - ".github/SECURITY.md"
+        - ".gitlab/SECURITY.md"
       message: "Security policy (SECURITY.md) required"
 
   - id: DOC-SEC-002