CM-038 Integrate Symfony Serializer Component

[ChernegaSergiy]

Implement Symfony Serializer for Standardized API and Data Export

User Story

As a developer, I want standardized serialization so API responses and data export are consistent.

Acceptance Criteria

1. Add symfony/serializer package to composer.json.
2. Implement normalizers for model classes.
3. Add serialization groups for different API endpoints.
4. Replace manual JSON encoding in exporters.
5. Add deserialization for API input validation.