perf(admin-log): add createdAt index for activity feed and retention policy

[Shrikant-sharma-9]

Summary

This PR adds a descending index on the createdAt field of the AdminLog model to optimize the primary query patterns used by the admin activity feed and the retention cleanup job.

Rationale

A review of the current AdminLog query patterns identified two primary workloads:

1. Activity Feed Retrieval

   AdminLog.find().sort('-createdAt')

   Retrieves all admin logs ordered by newest first.

2. Retention Policy Cleanup

   deleteMany({ createdAt: { $lt: before } })

   Removes log entries older than the configured retention period.

A descending index on createdAt allows MongoDB to efficiently satisfy both workloads by avoiding collection scans and in-memory sorting.

A compound index such as { adminId: 1, createdAt: -1 } was considered but rejected because neither query filters on adminId. Since MongoDB uses the leading fields of a compound index, that index would not benefit these workloads.

Verification

• ✅ npx tsc --noEmit
• ✅ npm run test

Both completed successfully.