Clean up SAM build workflow after debugging

- Use npm run build to ensure esbuild is on PATH
- Include hidden files for .aws-sam artifact upload
- Error if no artifacts found

Author: smessmer

.github/workflows/backend.yml

@@ -40,6 +40,8 @@ jobs:
         with:
           name: sam-build
           path: backend/.aws-sam
+          if-no-files-found: error
+          include-hidden-files: true
 
   deploy:
     needs: build