[SHD-1959] Reactive audience recalculation

[xjunior]

SHD-1359

This pull request introduces significant changes to the audiences context and criteria/group association logic, refactoring how group criteria are handled, stored, and queried. Here’s a summary of the main changes:

• Moves from storing group info as JSON directly on criteria to a proper join table (audiences_criterion_groups).
• Refactors queries, associations, and serialization to use ActiveRecord associations.
• Updates controller, model, and test code accordingly.
• Adds new database migrations for the join table and column rename.
• Cleans up and improves group membership notification logic.