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/Configuration/FileStorageOptions.cs

@@ -0,0 +1,9 @@
+namespace FabWorks.Api.Configuration
+{
+    public class FileStorageOptions
+    {
+        public const string SectionName = "FileStorage";
+
+        public string OutputFolder { get; set; } = @"C:\ExportDXF\Output";
+    }
+}