[OGUI-1620] Migration to layouts database - #3098

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This is a major PR that refactors the backend to fully integrate the layouts database for layout management. Key changes include:

• Layouts are now retrieved directly from the database instead of the db.json file.
• Repositories are introduced to provide an interface for interacting with Sequelize models.
• Business logic is encapsulated in services, which act as intermediaries between controllers and repositories.
• Controllers are updated to work with the new services, ensuring a cleaner and more maintainable flow.
• All validation logic has been moved to middleware, improving separation of concerns.
• Relevant endpoints have been updated to align with the new data structure and services.
• Tests added to cover both refactored and newly introduced code.
• Seeders are included to simulate realistic data scenarios for dev/test purposes.