[Docs] Add comprehensive API/backend documentation

[webmaxru]

Summary

Create comprehensive documentation for the Express REST API backend to help developers understand the codebase structure, setup, and usage.

Documentation Scope

The following areas should be documented:

1. API Overview

• Architecture overview (Express + SQLite + repository pattern)
• Directory structure explanation (api/src/)
• Configuration and environment variables

2. Database Documentation

• SQLite schema and migrations (api/sql/migrations/)
• Seed data structure (api/sql/seed/)
• Database configuration (api/src/db/)

3. Data Models

• Document all models in api/src/models/:
  • Branch, Delivery, Headquarters, Order, OrderDetail
  • OrderDetailDelivery, Product, Supplier

4. API Endpoints

• Document all REST endpoints from api/src/routes/:
  • Branch, Delivery, Headquarters, Order, OrderDetail
  • OrderDetailDelivery, Product, Supplier
• Request/response formats
• Error handling patterns

5. Repository Layer

• Document repository pattern implementation
• CRUD operations for each entity

6. Development Guide

• Local setup instructions
• Running tests
• Building and deployment

Acceptance Criteria

• README.md updated with quick start guide
• API endpoint documentation (consider OpenAPI/Swagger expansion)
• Database schema documentation
• Code examples for common operations
• Error handling documentation

Additional Context

Current Swagger documentation exists at api/api-swagger.json - this can be expanded and integrated into the documentation.