fix(redis): configure maxmemory and eviction policy to prevent OOM

Redis had no maxmemory set, causing the Prometheus alert expression
(used/max) to evaluate to +Inf. Set maxmemory to 100mb with allkeys-lru
eviction policy, and guard the alert expression against division by zero.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -23,6 +23,7 @@ services:
   redis:
     image: redis:7-alpine
     restart: always
+    command: redis-server --maxmemory 100mb --maxmemory-policy allkeys-lru
     ports:
       - "6380:6379"  # Use 6380 to avoid conflict with host Redis
     mem_limit: 128m