chore(deps): bump email-validator from 2.1.0.post1 to 2.3.0 in /backend

Bumps [email-validator](https://github.com/JoshData/python-email-validator) from 2.1.0.post1 to 2.3.0.
- [Release notes](https://github.com/JoshData/python-email-validator/releases)
- [Changelog](https://github.com/JoshData/python-email-validator/blob/main/CHANGELOG.md)
- [Commits](https://github.com/JoshData/python-email-validator/commits/v2.3.0)

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

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

backend/requirements.txt

@@ -25,7 +25,7 @@ stripe==15.0.1
 aiosmtplib==5.1.0
 aiohttp==3.13.4  # Updated: Fixed zip bomb, DoS, and directory traversal vulnerabilities (was 3.9.1)
 aioimaplib==2.0.1
-email-validator==2.1.0.post1
+email-validator==2.3.0
 
 # Gmail API (for direct email injection)
 google-api-python-client==2.193.0