chore(deps): bump aiosmtplib from 3.0.1 to 5.1.0 in /backend

Bumps [aiosmtplib](https://github.com/cole/aiosmtplib) from 3.0.1 to 5.1.0.
- [Release notes](https://github.com/cole/aiosmtplib/releases)
- [Changelog](https://github.com/cole/aiosmtplib/blob/main/CHANGELOG.rst)
- [Commits](https://github.com/cole/aiosmtplib/compare/v3.0.1...v5.1.0)

---
updated-dependencies:
- dependency-name: aiosmtplib
  dependency-version: 5.1.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

backend/requirements.txt

@@ -22,7 +22,7 @@ httpx==0.28.1
 stripe==14.4.1
 
 # Email & Mail Processing
-aiosmtplib==3.0.1
+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