Uniqueness validation for complex keys

この issue には説明がありません。

The issue title suggests adding uniqueness validation for complex keys across multiple database adapters. Start by examining the validation logic in the ORM's core, likely in lib/validations.js or similar. Look at how existing uniqueness validations work for simple keys and understand the adapter interfaces for each supported database. 'Done' means the validation works consistently across redis, mongodb, mysql, sqlite3, postgresql, and arango.

索引モデルが issue の本文から書いたものです。