initial commit

backend/Domain/Entities/MaterialModifier.cs

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+using Extrudex.Domain.Base;
+
+namespace Extrudex.Domain.Entities;
+
+/// <summary>
+/// Optional modifier/additive for a material (e.g., "Carbon Fiber", "Glass Fiber",
+/// "Wood Fill", "Glow-in-the-Dark"). Not every spool has a modifier.
+/// </summary>
+public class MaterialModifier : AuditableEntity
+{
+    /// <summary>
+    /// Human-readable name of the modifier (e.g., "Carbon Fiber", "Wood Fill").
+    /// </summary>
+    public string Name { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Foreign key to the parent MaterialBase. A modifier belongs to exactly one base material.
+    /// </summary>
+    public Guid MaterialBaseId { get; set; }
+
+    /// <summary>
+    /// Navigation to the parent MaterialBase.
+    /// </summary>
+    public MaterialBase MaterialBase { get; set; } = null!;
+
+    /// <summary>
+    /// Navigation collection of spools with this modifier.
+    /// </summary>
+    public ICollection<Spool> Spools { get; set; } = new List<Spool>();
+}