Define API LLD and contract for Cargo Management integration

Description:

• Establish a Low-Level Design (LLD) to align frontend (Vue.js) and backend (Spring Boot) on API contracts prior to implementation.
• Provide a clear call flow illustrating interactions across Controller, Service, Repository, and database layers for typical requests.
• Specify a concrete endpoint for creating cargo entries, including exact request/response JSON formats and headers.
• Standardize error response structures for validation, missing resources, and server errors to ensure predictable handling on the frontend.

Deliverables:

• Sequence diagram clarity: Provide a sequence diagram showing end-to-end flow for a successful API call.
• Endpoint specification: Define POST /api/cargo with required fields, success response (201), and Location header.
• JSON contracts: Document exact request and response JSON schemas, including sample payloads and timestamps.
• Error standards: Define standardized 400/404/500 response bodies with fields like timestamp, status, error, and message.