Refactor ASP.NET Core codebase for cleaner architecture and performance #3

Description:
Improve code organization, reduce duplication, and optimize database queries for better maintainability.

Tasks:

• Extract business logic from controllers into services
• Add global exception handling middleware
• Optimize Entity Framework queries (fix N+1 issues)
• Remove magic strings (use nameof, constants)
• Add cancellation token support to async methods

Acceptance Criteria:

• Controllers are thin (only handle HTTP)
• Consistent error responses across endpoints
• Improved query performance (measured)