Introduce cold storage for saving storage costs

To save storage cost, we are going to set the TTL attribute to 90 days away when we're soft-deleting an item so it will be automatically deleted by dynamoDB without extra cost.
cc @jsecretan

ref: https://aws.amazon.com/blogs/aws/new-manage-dynamodb-items-using-time-to-live-ttl/

Start by locating the soft-delete path and the DynamoDB persistence configuration in the Go server. Check how deleted items are represented and where TTL attributes can be set; done means soft-deleted items receive a 90-day TTL and existing behavior remains covered by the available tests.

Written by the indexing model from the issue text.