initial commit

backend/Domain/Entities/MaterialBase.cs

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+using Extrudex.Domain.Base;
+
+namespace Extrudex.Domain.Entities;
+
+/// <summary>
+/// Base polymer/material type. This is a lookup table enforcing consistent
+/// material naming across all spools. Free-text material names are not allowed.
+/// </summary>
+public class MaterialBase : AuditableEntity
+{
+    /// <summary>
+    /// Human-readable name of the base material (e.g., "PLA", "PETG", "ABS").
+    /// </summary>
+    public string Name { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Density of the material in g/cm³ (g/mL). Used for deriving grams consumed
+    /// from mm extruded: grams = mm × cross_section_area × density.
+    /// </summary>
+    public decimal DensityGperCm3 { get; set; }
+
+    /// <summary>
+    /// Navigation collection of finishes available for this material base.
+    /// </summary>
+    public ICollection<MaterialFinish> Finishes { get; set; } = new List<MaterialFinish>();
+
+    /// <summary>
+    /// Navigation collection of modifiers applicable to this material base.
+    /// </summary>
+    public ICollection<MaterialModifier> Modifiers { get; set; } = new List<MaterialModifier>();
+
+    /// <summary>
+    /// Navigation collection of spools made from this material base.
+    /// </summary>
+    public ICollection<Spool> Spools { get; set; } = new List<Spool>();
+}