Create index for sims.notifications table with dateSent and createdAt

**Describe the task**
Create index for sims.notifications table with dateSent and createdAt as sims.notifications can grow in large numbers and will be retrieved every minute by the process-notification scheduler

**Acceptance Criteria**
- [ ] Create index for sims.notifications table with dateSent and createdAt
- [ ] Check on the query on getPendingCOEs

Start by locating the schema or migration for sims.notifications and the getPendingCOEs query used by the process-notification scheduler. Check how notifications are filtered by dateSent and createdAt, then verify that the requested index is present and that the query has been reviewed for the scheduler's once-per-minute retrieval.

Written by the indexing model from the issue text.