Keep a broken Redis from 500ing the whole API

Redis backs the DRF throttles, and the stock RedisCache raises inside the
throttle check when Redis is unreachable or refusing writes (a failed RDB
snapshot disables writes by default) — turning a cache problem into a
blanket 500, which is exactly what took prod down. ResilientRedisCache
treats backend failures as cache misses, logs the first one per worker, and
lets rate limits degrade until Redis is back.

backend/config/settings.py

@@ -242,7 +242,7 @@ SIMILARITY_TTL_MINUTES = int(os.getenv("SIMILARITY_TTL_MINUTES", "30"))
 # workers and management commands (e.g. the mirrored guest blacklist).
 CACHES = {
     "default": {
-        "BACKEND": "django.core.cache.backends.redis.RedisCache",
+        "BACKEND": "apps.core.cache.ResilientRedisCache",
         "LOCATION": os.getenv("REDIS_URL", "redis://127.0.0.1:6380/1"),
     }
 }