docker: add Redis service for shared cache and rate-limit buckets

Matches cache.handler=redis in .env.docker.example; lets the file-based
rate limiter and page cache work consistently across web/api/worker replicas.

docker-compose.yml

@@ -80,8 +80,21 @@ services:
       interval: 10s
       retries: 5
 
+  # Shared cache + rate-limit buckets across web/api/worker replicas.
+  redis:
+    image: redis:7-alpine
+    restart: unless-stopped
+    command: ["redis-server", "--maxmemory", "256mb", "--maxmemory-policy", "allkeys-lru"]
+    volumes:
+      - redis-data:/data
+    healthcheck:
+      test: ["CMD", "redis-cli", "ping"]
+      interval: 10s
+      retries: 5
+
 volumes:
   db-data:
+  redis-data:
   media-storage:
   incoming-storage:
   session-storage: