Add default content guard auto-assignment for distributions within a domain #7988

Summary

Add a default_content_guard field to the Domain model, so that when a new distribution is created within that domain, the specified content guard is automatically assigned to it.

Requirements

1. default_content_guard on Domain — Add a default_content_guard foreign key field to the Domain model that references a content guard. This is an optional field (nullable).
2. Auto-assignment on distribution creation — When a distribution is created within a domain that has a default_content_guard set, automatically assign that content guard to the new distribution.
3. Composite content guard for multiple guards — If users need multiple content guards applied by default, they should create a composite content guard and set it as the domain's default_content_guard.

Acceptance Criteria

• The Domain model has a default_content_guard field
• The default_content_guard can be set, updated, and cleared on a domain
• Creating a distribution in a domain with a default_content_guard automatically assigns it to the new distribution
• A composite content guard can be used as the default_content_guard to apply multiple guards