feat: add file storage service with content-addressed blob store

Add FileStorageService for DXF/PDF storage using content hashing,
FileStorageOptions config, FilesController for uploads, and
FileBrowserController for browsing stored files.

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

FabWorks.Api/DTOs/FileUploadResponse.cs

@@ -0,0 +1,11 @@
+namespace FabWorks.Api.DTOs
+{
+    public class FileUploadResponse
+    {
+        public string StoredFilePath { get; set; }  // kept for client compat, contains logical filename
+        public string ContentHash { get; set; }
+        public string FileName { get; set; }
+        public bool WasUnchanged { get; set; }
+        public bool IsNewFile { get; set; }
+    }
+}