[Backend] Support pipeline based scans
### Summary
Security scan profiles with `merge_request_pipeline` or `default_branch_pipeline` triggers should automatically add scan jobs to pipelines and enable pipelines when no other configuration source exists.
### Requirements
- Security scan profiles should add scan jobs to pipelines when configured with:
- `merge_request_pipeline` trigger
- `default_branch_pipeline` trigger
- Pipelines should be automatically enabled if no other source is present:
- No CI file configuration
- No security policy configuration
- No Auto DevOps configuration
### Scope
**Phase 1 Focus: SAST**
- Priority scanner: `gitlab-advanced-sast`
### Expected Behavior
When a security scan profile is configured with the specified triggers and no other pipeline sources exist, we should:
1. Add the appropriate SAST scan jobs to the pipeline
2. Enable pipeline execution
3. Prioritize using the `gitlab-advanced-sast` scanner over other SAST scanners
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_Description was generated using AI_
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