feat(docker): remove bundled Redis; use external host install

Compose stack is MySQL+Nexus only; wizard Redis check is non-blocking
and defaults to host.docker.internal for self-managed Redis.

Co-authored-by: Cursor <cursoragent@cursor.com>

docker/.env.prod.example

@@ -21,5 +21,9 @@ NEXUS_API_BASE_URL=https://api.synaglobal.vip
 NEXUS_DB_POOL_SIZE=30
 NEXUS_DB_MAX_OVERFLOW=20
 
+# Redis 不在 Docker 栈内 — 请在宿主机或 1Panel 应用商店自行安装
+# Nexus 容器通过 host.docker.internal 访问宿主机 Redis（见 docker-compose.prod.yml）
+NEXUS_REDIS_URL=redis://host.docker.internal:6379/0
+
 # Optional
 # NEXUS_PUBLISH_PORT=8600