Optimize CODEOWNERS group validation

What does this MR do and why?

Optimizes codeowners group validation by:

• Replacing eager loading with db-only queries using user IDs
• Batching group eligibility checks into a single query instead of N queries (one per group)
• Fetching eligible project members once and reusing for all groups
• Adding logging to track user ID counts for monitoring

References

Optimize validation of groups to ensure we are ... (#549621 - closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.