chore: add format:backend npm script to match ci

.github/workflows/format-backend.yaml

@@ -33,7 +33,7 @@ jobs:
           pip install black
 
       - name: Format backend
-        run: black . --exclude "/venv/"
+        run: npm run format:backend
 
       - name: Check for changes after format
         run: git diff --exit-code

package.json

@@ -13,6 +13,7 @@
 		"lint:types": "npm run check",
 		"lint:backend": "pylint backend/",
 		"format": "prettier --plugin-search-dir --write '**/*.{js,ts,svelte,css,md,html,json}'",
+		"format:backend": "black . --exclude \"/venv/\"",
 		"i18n:parse": "i18next --config i18next-parser.config.ts && prettier --write 'src/lib/i18n/**/*.{js,json}'"
 	},
 	"devDependencies": {