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

monitoring/prometheus/alert.rules.yml

@@ -140,7 +140,7 @@ groups:
           description: "Redis exporter cannot connect to Redis for 1 minute. Background tasks (emails, Celery) are not processing."
 
       - alert: RedisHighMemoryUsage
-        expr: redis_memory_used_bytes / redis_memory_max_bytes * 100 > 80
+        expr: redis_memory_max_bytes > 0 and redis_memory_used_bytes / redis_memory_max_bytes * 100 > 80
         for: 5m
         labels:
           severity: warning