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/appsettings.json

@@ -8,5 +8,8 @@
   "AllowedHosts": "*",
   "ConnectionStrings": {
     "FabWorksDb": "Server=localhost;Database=ExportDxfDb;Trusted_Connection=True;TrustServerCertificate=True;"
+  },
+  "FileStorage": {
+    "OutputFolder": "C:\\ExportDXF\\Output"
   }
 }