implemnet customer master used for both B2B and B2C.

Backend/ERPCore/Domain/Entities/Customer.cs

@@ -0,0 +1,38 @@
+using ERPCore.Domain.Enums;
+
+namespace ERPCore.Domain.Entities;
+
+/// <summary>
+/// Customer master for both B2B and B2C sales.
+/// Phase 1 keeps this lean: identity, contact, tax, credit, and default warehouse.
+/// </summary>
+public class Customer
+{
+    public int CustomerId { get; set; }
+    public string CustomerCode { get; set; } = string.Empty;
+    public CustomerType CustomerType { get; set; } = CustomerType.B2C;
+
+    public string Name { get; set; } = string.Empty;
+    public string? DisplayName { get; set; }
+    public string? Phone { get; set; }
+    public string? Email { get; set; }
+
+    public string? AddressLine1 { get; set; }
+    public string? AddressLine2 { get; set; }
+    public string? City { get; set; }
+    public string? Country { get; set; }
+
+    public string? TaxRegistrationNo { get; set; }
+    public decimal CreditLimit { get; set; }
+    public int CreditDays { get; set; }
+
+    public int? DefaultWarehouseId { get; set; }
+    public Warehouse? DefaultWarehouse { get; set; }
+
+    public EntityStatus Status { get; set; } = EntityStatus.Active;
+    public DateTime CreatedAt { get; set; }
+    public DateTime? UpdatedAt { get; set; }
+
+    /// <summary>PostgreSQL xmin-backed optimistic concurrency token (ETag source).</summary>
+    public uint RowVersion { get; set; }
+}

Backend/ERPCore/Domain/Enums/CustomerType.cs

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+namespace ERPCore.Domain.Enums;
+
+public enum CustomerType
+{
+    B2B = 1,
+    B2C = 2,
+    WalkIn = 3
+}