Add Redis-backed chat queue

ai_knowledge_base_v2/apps/backend/.env.example

@@ -4,6 +4,7 @@ DEBUG=true
 API_PREFIX=/api
 
 DATABASE_URL=mysql+pymysql://ai_kb:ai_kb@127.0.0.1:3306/ai_knowledge_base_v2?charset=utf8mb4
+REDIS_URL=
 AUTO_CREATE_TABLES=false
 
 JWT_SECRET_KEY=change-me-in-production
@@ -27,6 +28,7 @@ DEFAULT_USER_NAME_PREFIX=用户
 CHAT_MAX_ACTIVE_REQUESTS=2
 CHAT_MAX_QUEUE_SIZE=20
 CHAT_QUEUE_TIMEOUT_SECONDS=60
+CHAT_ACTIVE_LEASE_SECONDS=900
 
 BOOTSTRAP_ADMIN_USERNAME=admin
 BOOTSTRAP_ADMIN_PASSWORD=admin123456