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>

.gitlab/merge_request_templates/default.md

@@ -0,0 +1,41 @@
+## Summary
+
+<!-- Brief description of what this MR does -->
+
+## Changes
+
+<!-- List the main changes -->
+-
+
+## Type of Change
+
+<!-- Check all that apply -->
+- [ ] Bug fix (non-breaking change that fixes an issue)
+- [ ] New feature (non-breaking change that adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to change)
+- [ ] Documentation update
+- [ ] Refactoring (no functional changes)
+
+## Testing
+
+<!-- Describe how you tested these changes -->
+- [ ] Unit tests pass (`pytest tests/`)
+- [ ] Architecture validation passes (`python scripts/validate_architecture.py`)
+- [ ] Manual testing performed
+
+## Checklist
+
+- [ ] My code follows the project's coding standards
+- [ ] I have updated the documentation accordingly
+- [ ] I have added tests that prove my fix/feature works
+- [ ] All new and existing tests pass
+- [ ] No new warnings are introduced
+
+## Related Issues
+
+<!-- Link any related issues -->
+Closes #
+
+## Screenshots
+
+<!-- If applicable, add screenshots -->