Feat/user svc

[ClaudiaDuenzinger]

🚀 User Service Implementation - Complete API Compliance & Error Handling

** 📋 Pull Request Summary**

• This PR implements a complete, production-ready User Service that fully complies with the OpenAPI specification and provides error handling. The service now supports user registration, authentication, profile management, and comprehensive error responses.

�� Authentication & Security

• JWT-based authentication with secure token validation
• Password hashing using BCrypt for security
• Stateless session management for scalability
• Custom authentication entry point for structured error responses

👤 User Management

• User registration with duplicate email validation
• Profile retrieval with complete user data including preferences
• Profile updates with partial update support
• User preferences management (event format, industry, language, timezone)
• Login/logout functionality with JWT tokens

🛡️ Error Handling

• Structured ErrorResponse objects matching OpenAPI specification
• Comprehensive error types: MISSING_TOKEN, INVALID_TOKEN, INVALID_CREDENTIALS, USER_ALREADY_EXISTS, USER_NOT_FOUND
• Proper HTTP status codes (401, 404, 409, 500)
• Global exception handler for consistent error formatting
• Custom business exceptions for specific error scenarios

��️ Data Management

• PostgreSQL integration with JPA/Hibernate
• Complete user entity with all required fields
• Timestamp management (createdAt, updatedAt, lastLoginAt)
• User preferences as embeddable entity
• Proper DTO mapping between entities and API models