[PR-for-TASK] Implement C# POCO data model for eCommerce catalog and order system with Entity Framework

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This PR implements a complete C# POCO (Plain Old CLR Object) data model for an eCommerce catalog and order system using Entity Framework Core. The implementation provides a production-ready foundation for eCommerce applications with proper relationships, constraints, and Entity Framework configuration.

Implementation Overview

The data model consists of 5 core entities that represent a typical eCommerce system:

• Category - Product categorization with unique names and descriptions
• Product - Catalog items with pricing, stock quantity, SKU tracking, and category relationships
• Customer - User profiles with contact information and shipping addresses
• Order - Purchase orders with status tracking, shipping details, and customer relationships
• OrderItem - Order line items that create many-to-many relationships between orders and products

Key Features

Entity Framework Configuration

• Complete DbContext implementation with Fluent API configuration
• Proper foreign key relationships with appropriate cascade/restrict rules
• Unique constraints on business-critical fields (Email, SKU, OrderNumber)
• Composite indexes for query performance optimization
• Default values for timestamps and status fields

Data Validation & Integrity

• Data annotations for field validation (Required, MaxLength, EmailAddress)
• Decimal precision configuration for monetary values (18,2)
• Navigation properties for Entity Framework relationship traversal
• Seed data for testing and demonstration purposes

Database Support

• SQL Server provider configuration with migration support
• In-memory database support for testing and demos
• Design-time factory for Entity Framework tooling

Demo Application

The included console application demonstrates:

• Database creation and seeding
• Querying entities with relationships using LINQ
• Creating complex orders with multiple items
• Proper Entity Framework change tracking and persistence

// Example: Creating an order with multiple items
var order = new Order
{
    OrderNumber = $"ORD-{DateTime.Now.Ticks}",
    CustomerId = customer.Id,
    TotalAmount = orderItems.Sum(oi => oi.TotalPrice),
    OrderItems = orderItems
};

Documentation

Complete documentation includes:

• Detailed README with entity descriptions and relationships
• Entity Relationship Diagram showing all table relationships
• Usage instructions for both development and production scenarios
• Migration guidelines for database deployment

The implementation follows Entity Framework best practices and provides a solid foundation that can be easily extended for additional eCommerce features like shopping carts, payment processing, or inventory management.

Fixes #2.

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