Implement `:upsert` action

[rauann]

Summary

Implement explicit upsert mode for create/update operations.

Context

Currently, create operations use PutItem with a uniqueness check, and updates use UpdateItem with an existence check. An upsert mode would allow inserting a new item or updating an existing one in a single operation without requiring the caller to know if the item exists.

Implementation Notes

• DynamoDB's PutItem naturally performs upsert behavior (overwrites if exists)
• May need a new action type or option to enable upsert semantics
• Consider how to handle attribute merging vs full replacement

References

• DynamoDB PutItem