build(deps-dev): bump @types/nodemailer in /backend (#223)

Bumps [@types/nodemailer](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/HEAD/types/nodemailer) from 6.4.21 to 7.0.10.
- [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: 7.0.10
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

backend/package.json

@@ -37,7 +37,7 @@
   "devDependencies": {
     "@biomejs/biome": "^2.4.1",
     "@types/node": "^25.2.3",
-    "@types/nodemailer": "^6.4.21",
+    "@types/nodemailer": "^7.0.10",
     "@types/supertest": "^6.0.2",
     "@vitest/coverage-v8": "^4.0.18",
     "drizzle-kit": "^0.31.9",