Restrict @mentions only to group members (self-hosted version)

In the self-hosted version of Outline, when a user types @ to mention someone, the suggestion list displays all members of the workspace. This can be problematic in large organizations or environments with multiple distinct teams, as users may accidentally mention colleagues who are not part of their specific group or project. Problem: We need a functionality that restricts mentions so that users can only mention people who are members of the same group they belong to. The idea is that the autocomplete list triggered by @ should filter and display exclusively the members of the group(s) to which the author of the document or comment belongs. Request: Add a configuration option (potentially at the group level or workspace level) that allows administrators to limit @mention suggestions to only the members of the current user's group(s). Use Case: • Scenario: A workspace with several departments (e.g., Engineering, Marketing, Sales). • Action: A user from th…