chore(deps): bump redis from 7.3.0 to 7.4.0 in /backend

Bumps [redis](https://github.com/redis/redis-py) from 7.3.0 to 7.4.0.
- [Release notes](https://github.com/redis/redis-py/releases)
- [Changelog](https://github.com/redis/redis-py/blob/master/CHANGES)
- [Commits](https://github.com/redis/redis-py/compare/v7.3.0...v7.4.0)

---
updated-dependencies:
- dependency-name: redis
  dependency-version: 7.4.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

backend/requirements.txt

@@ -35,7 +35,7 @@ google-auth-httplib2==0.3.0
 
 # Job Queue & Cache
 celery==5.6.2
-redis==7.3.0
+redis==7.4.0
 
 # Security & Encryption
 cryptography==46.0.6  # Updated: Fixed NULL pointer dereference (was 42.0.0)