chore(deps-dev): bump svelte from 5.1.4 to 5.1.9 in /backend

Bumps [svelte](https://github.com/sveltejs/svelte/tree/HEAD/packages/svelte) from 5.1.4 to 5.1.9.
- [Release notes](https://github.com/sveltejs/svelte/releases)
- [Changelog](https://github.com/sveltejs/svelte/blob/main/packages/svelte/CHANGELOG.md)
- [Commits](https://github.com/sveltejs/svelte/commits/svelte@5.1.9/packages/svelte)

---
updated-dependencies:
- dependency-name: svelte
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

backend/package.json

@@ -72,7 +72,7 @@
         "prettier-plugin-svelte": "^3.2.7",
         "prettier-plugin-tailwindcss": "^0.6.8",
         "sass": "^1.80.6",
-        "svelte": "^5.1.4",
+        "svelte": "^5.1.9",
         "svelte-file-dropzone": "^2.0.9",
         "svelte-gestures": "^5.0.6",
         "svelte-preprocess": "^6.0.3",