feat: add Drawing entity with revision tracking

Introduce a Drawing table that tracks unique drawing numbers with their
current PDF content hash and revision counter. ExportRecord gains a
DrawingId FK. Includes a data migration to seed Drawings from existing
export records.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

FabWorks.Core/Migrations/20260220125334_AddDrawingEntity.cs

@@ -0,0 +1,75 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace FabWorks.Core.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddDrawingEntity : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<int>(
+                name: "DrawingId",
+                table: "ExportRecords",
+                type: "int",
+                nullable: true);
+
+            migrationBuilder.CreateTable(
+                name: "Drawings",
+                columns: table => new
+                {
+                    Id = table.Column<int>(type: "int", nullable: false)
+                        .Annotation("SqlServer:Identity", "1, 1"),
+                    DrawingNumber = table.Column<string>(type: "nvarchar(100)", maxLength: 100, nullable: true),
+                    Title = table.Column<string>(type: "nvarchar(200)", maxLength: 200, nullable: true),
+                    PdfContentHash = table.Column<string>(type: "nvarchar(64)", maxLength: 64, nullable: true),
+                    Revision = table.Column<int>(type: "int", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_Drawings", x => x.Id);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_ExportRecords_DrawingId",
+                table: "ExportRecords",
+                column: "DrawingId");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_Drawings_DrawingNumber",
+                table: "Drawings",
+                column: "DrawingNumber",
+                unique: true,
+                filter: "[DrawingNumber] IS NOT NULL");
+
+            migrationBuilder.AddForeignKey(
+                name: "FK_ExportRecords_Drawings_DrawingId",
+                table: "ExportRecords",
+                column: "DrawingId",
+                principalTable: "Drawings",
+                principalColumn: "Id",
+                onDelete: ReferentialAction.SetNull);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropForeignKey(
+                name: "FK_ExportRecords_Drawings_DrawingId",
+                table: "ExportRecords");
+
+            migrationBuilder.DropTable(
+                name: "Drawings");
+
+            migrationBuilder.DropIndex(
+                name: "IX_ExportRecords_DrawingId",
+                table: "ExportRecords");
+
+            migrationBuilder.DropColumn(
+                name: "DrawingId",
+                table: "ExportRecords");
+        }
+    }
+}