Implement ticket management and refactor exception handling

[shmoonwalker]

Summary

Implemented and improved core API functionality for users and tickets in the Ticket Tracking System.

This PR adds full CRUD support for users and tickets, ticket assignment functionality, DTO-based API responses, validation, and centralized error handling.

Changes

Users

• Added create, read, update, and delete functionality for users.
• Added request DTOs for user creation and update.
• Added UserResponse DTO to avoid returning internal model objects directly.
• Added duplicate email validation.
• Added proper not-found handling for missing users.

Tickets

• Added create, read, update, and delete functionality for tickets.
• Added request DTOs for ticket creation and update.
• Added TicketResponse DTO including assigned user IDs.
• Added validation for required fields such as title, project ID, and status.
• Added project existence validation before creating or updating tickets.
• Updated ticket responses to include real assigned user IDs from the user_ticket table.
• Moved response mapping logic into the service layer for cleaner controller responsibilities.

Ticket Assignees

• Added endpoint to assign a user to a ticket.
• Added endpoint to remove a user from a ticket.
• Added validation to check whether ticket and user exist before assignment.
• Added duplicate assignment prevention.
• Added database support through the user_ticket junction table.

Error Handling

• Added centralized GlobalExceptionHandler.

• Added structured error responses using ErrorResponse.

• Added handling for:

  • resource not found errors
  • duplicate email errors
  • validation errors
  • invalid request bodies
  • database constraint violations
  • illegal arguments such as duplicate ticket assignments

Testing

Tested with Postman:

• POST /users
• GET /users
• GET /users/{id}
• PUT /users/{id}
• DELETE /users/{id}
• POST /tickets
• GET /tickets
• GET /tickets/{id}
• PUT /tickets/{id}
• DELETE /tickets/{id}
• POST /tickets/{ticketId}/assignees/{userId}
• DELETE /tickets/{ticketId}/assignees/{userId}

Also tested error scenarios:

• creating a user with a duplicate email
• requesting non-existing users or tickets
• creating/updating tickets with invalid project IDs
• assigning the same user to the same ticket twice
• sending invalid request bodies
• sending invalid validation data