feat: add export history auto-fill, fix filename prefixes, persist records for all doc types

- Add database-first lookup for equipment/drawing number auto-fill when
  reopening previously exported files
- Remove prefix prepending for named parts (only use prefix for PT## BOM items)
- Create ExportRecord/BomItem/CutTemplate chains for Part and Assembly
  exports, not just Drawings
- Add auto-incrementing item numbers across drawing numbers
- Add content hashing (SHA256) for DXF and PDF versioning with
  stash/archive pattern
- Add EF Core initial migration for ExportDxfDb

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

ExportDXF/Models/ExportContext.cs

@@ -32,6 +32,16 @@ namespace ExportDXF.Services
         /// </summary>
         public string FilePrefix { get; set; }
 
+        /// <summary>
+        /// Equipment number from the UI (e.g., "5028").
+        /// </summary>
+        public string Equipment { get; set; }
+
+        /// <summary>
+        /// Drawing number from the UI (e.g., "A02", "Misc").
+        /// </summary>
+        public string DrawingNo { get; set; }
+
         /// <summary>
         /// Selected Equipment ID for API operations (optional).
         /// </summary>

ExportDXF/Models/ExportRecord.cs

@@ -11,6 +11,7 @@ namespace ExportDXF.Models
         public string OutputFolder { get; set; }
         public DateTime ExportedAt { get; set; }
         public string ExportedBy { get; set; }
+        public string PdfContentHash { get; set; }
 
         public virtual ICollection<BomItem> BomItems { get; set; } = new List<BomItem>();
     }

ExportDXF/Models/Item.cs

@@ -61,5 +61,15 @@ namespace ExportDXF.Services
         /// The SolidWorks component reference.
         /// </summary>
         public Component2 Component { get; set; }
+
+        /// <summary>
+        /// SHA256 content hash of the exported DXF (transient, not persisted).
+        /// </summary>
+        public string ContentHash { get; set; }
+
+        /// <summary>
+        /// Path to the stashed (backed-up) previous DXF file (transient, not persisted).
+        /// </summary>
+        public string StashedFilePath { get; set; }
     }
 }