build(deps): bump redis from 7.4.0 to 8.1.0 in /backend

Bumps [redis](https://github.com/redis/redis-py) from 7.4.0 to 8.1.0.
- [Release notes](https://github.com/redis/redis-py/releases)
- [Changelog](https://github.com/redis/redis-py/blob/master/CHANGES)
- [Commits](redis/redis-py@v7.4.0...v8.1.0)

---
updated-dependencies:
- dependency-name: redis
  dependency-version: 8.1.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

backend/pyproject.toml

@@ -24,7 +24,7 @@ dependencies = [
     "svix>=1.99.1",
     "clerk-backend-api>=5.0.6",
     "slowapi>=0.1.10",
-    "redis>=5.0.0",
+    "redis>=8.1.0",
     "websockets>=16.1.1",
     "fastmcp>=3.4.7",
     "sentry-sdk[fastapi]>=2.68.0",