feat(knowledge): add short-term ask memory

backend/script/sql/update/aihr_20260718_knowledge_conversation_mysql8.sql

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+-- 问师傅短期多轮会话。只保存最近六轮的脱敏截断 JSON，30 分钟不活跃过期。
+
+CREATE TABLE IF NOT EXISTS `aihr_knowledge_conversation` (
+  `id` bigint NOT NULL AUTO_INCREMENT COMMENT '主键',
+  `tenant_id` varchar(20) NOT NULL COMMENT '租户编号',
+  `app_id` bigint NOT NULL COMMENT '内部调用应用ID',
+  `user_id` bigint NOT NULL COMMENT '内部用户ID',
+  `conversation_id` varchar(64) NOT NULL COMMENT '客户端随机会话ID',
+  `version` bigint NOT NULL DEFAULT 0 COMMENT '乐观并发版本',
+  `context_json` json DEFAULT NULL COMMENT '最近六轮脱敏短期上下文',
+  `expires_time` datetime NOT NULL COMMENT '不活跃过期时间',
+  `create_time` datetime NOT NULL COMMENT '创建时间',
+  `update_time` datetime NOT NULL COMMENT '更新时间',
+  PRIMARY KEY (`id`),
+  UNIQUE KEY `uk_aihr_knowledge_conversation` (`tenant_id`, `app_id`, `user_id`, `conversation_id`),
+  KEY `idx_aihr_knowledge_conversation_expire` (`tenant_id`, `expires_time`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci COMMENT='知识问答短期会话';