🔧(backend) fix Redis dependency conflict by downgrading to v5.2.1

Resolve ResolutionImpossible error where meet requires redis==6.2.0
but kombu[redis] 5.5.x requires redis<=5.2.1. Downgrade to redis==5.2.1
satisfies overlapping constraints and testing confirms compatibility.

Pin the redis dependency.

renovate.json

@@ -7,7 +7,7 @@
       "enabled": false,
       "groupName": "ignored python dependencies",
       "matchManagers": ["pep621"],
-      "matchPackageNames": []
+      "matchPackageNames": ["redis"]
     },
     {
       "enabled": false,