Add description field to CI/CD Catalog Component (backend)

What does this MR do and why?

Adds an optional description field to the CI/CD Catalog Component spec (backend only). This allows component authors to provide a short description directly in the YAML file, which can then be exposed via the GraphQL API.

Previously, component descriptions had to be documented separately in the README. This change enables inline documentation that can be automatically shown in the Catalog UI.

Note: Frontend changes to display the description in the UI are in a separate follow-up MR for multi-version compatibility.

References

• Relates to #588286
• Follow-up frontend MR: !222883

How to set up and validate locally

1. Create a new project with a component in templates/my-component.yml that includes spec: description: "Your description"
2. Add a README.md and project description
3. Go to Settings > General > Visibility, project features, permissions
4. Enable CI/CD Catalog project
5. Create a release with a semantic version tag (e.g., 1.0.0)
6. Query the component via GraphQL and verify the description field is returned

MR acceptance checklist

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