feat: Add MaterialStockLength entity for inventory tracking

Introduces a new entity to track available stock lengths per material,
enabling in-stock vs. purchase-needed distinction during optimization.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CutList.Web/Migrations/20260202033321_AddMaterialStockLengths.cs

@@ -0,0 +1,49 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace CutList.Web.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddMaterialStockLengths : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "MaterialStockLengths",
+                columns: table => new
+                {
+                    Id = table.Column<int>(type: "int", nullable: false)
+                        .Annotation("SqlServer:Identity", "1, 1"),
+                    MaterialId = table.Column<int>(type: "int", nullable: false),
+                    LengthInches = table.Column<decimal>(type: "decimal(10,4)", precision: 10, scale: 4, nullable: false),
+                    Notes = table.Column<string>(type: "nvarchar(255)", maxLength: 255, nullable: true),
+                    IsActive = table.Column<bool>(type: "bit", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_MaterialStockLengths", x => x.Id);
+                    table.ForeignKey(
+                        name: "FK_MaterialStockLengths_Materials_MaterialId",
+                        column: x => x.MaterialId,
+                        principalTable: "Materials",
+                        principalColumn: "Id",
+                        onDelete: ReferentialAction.Cascade);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_MaterialStockLengths_MaterialId_LengthInches",
+                table: "MaterialStockLengths",
+                columns: new[] { "MaterialId", "LengthInches" },
+                unique: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "MaterialStockLengths");
+        }
+    }
+}