chore(deps): bump stripe from 14.4.1 to 15.0.1 in /backend

Bumps [stripe](https://github.com/stripe/stripe-python) from 14.4.1 to 15.0.1.
- [Release notes](https://github.com/stripe/stripe-python/releases)
- [Changelog](https://github.com/stripe/stripe-python/blob/master/CHANGELOG.md)
- [Commits](https://github.com/stripe/stripe-python/compare/v14.4.1...v15.0.1)

---
updated-dependencies:
- dependency-name: stripe
  dependency-version: 15.0.1
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

backend/requirements.txt

@@ -19,7 +19,7 @@ authlib==1.6.9  # Updated: Fixed OIDC hash binding, JWE RSA1_5 padding oracle, a
 httpx==0.28.1
 
 # Payment Processing
-stripe==14.4.1
+stripe==15.0.1
 
 # Email & Mail Processing
 aiosmtplib==3.0.1