build(deps-dev): bump @types/nodemailer from 7.0.11 to 8.0.0 in /backend

Bumps [@types/nodemailer](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/HEAD/types/nodemailer) from 7.0.11 to 8.0.0.
- [Release notes](https://github.com/DefinitelyTyped/DefinitelyTyped/releases)
- [Commits](https://github.com/DefinitelyTyped/DefinitelyTyped/commits/HEAD/types/nodemailer)

---
updated-dependencies:
- dependency-name: "@types/nodemailer"
  dependency-version: 8.0.0
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Daniel Volz <mail@danielvolz.org>

backend/package.json

@@ -41,7 +41,7 @@
   "devDependencies": {
     "@biomejs/biome": "^2.4.10",
     "@types/node": "^25.5.2",
-    "@types/nodemailer": "^7.0.11",
+    "@types/nodemailer": "^8.0.0",
     "@types/supertest": "^7.2.0",
     "@vitest/coverage-v8": "^4.1.2",
     "drizzle-kit": "^0.31.10",