Refactor Configuration System to Use Value Objects

[frogr]

Problem

The Hub::Config system uses complex nested attributes with ActiveModel, making it hard to understand and test.

Tasks

• Replace nested attribute classes with simple value objects
• Implement configuration builder pattern
• Extract validation to separate validator classes
• Simplify persistence logic

Acceptance Criteria

• Create app/models/configuration/ directory with value objects
• Remove ActiveModel dependencies from config classes
• Configuration validation extracted to dedicated classes
• All configuration specs pass
• Configuration loading/saving remains functional
• Add unit tests for value objects

Files to modify

• lib/hub/config.rb and all files in lib/hub/config/
• app/services/configuration_persistence_service.rb
• app/services/configuration_update_service.rb

Phase: 2 (Domain)
Priority: High