chore(deps): update email-validator requirement in /backend

Updates the requirements on [email-validator](https://github.com/JoshData/python-email-validator) to permit the latest version.
- [Release notes](https://github.com/JoshData/python-email-validator/releases)
- [Changelog](https://github.com/JoshData/python-email-validator/blob/main/CHANGELOG.md)
- [Commits](JoshData/python-email-validator@v2.0.0...v2.3.0)

---
updated-dependencies:
- dependency-name: email-validator
  dependency-version: 2.3.0
  dependency-type: direct:production
...

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Author: dependabot[bot]

backend/requirements.txt

@@ -5,7 +5,7 @@ httpx>=0.27.0
 python-dotenv>=1.0.0
 pydantic>=2.0.0
 pydantic-settings>=2.0.0
-email-validator>=2.0.0  # Required for EmailStr validation
+email-validator>=2.3.0  # Required for EmailStr validation
 
 # WebSocket support
 websockets>=13.0