Add getMutualReactions Method

[Silent-Watcher]

Feature: Add getMutualReactions Method

Description:
Add a method getMutualReactions(reactableId, userId1, userId2) that returns which reactions two users have in common on a specific document.

Acceptance Criteria:

• Method exists: Model.getMutualReactions(reactableId, userId1, userId2).
• Returns an array of reaction types that both users have applied to the document.
• Is efficiently implemented using a single query.
• Method is fully typed.
• Tests are added for this method.
• Add usage examples in docs