rag snippet生成入库和写rag

file/tableschema/rag_snippet.sql

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+CREATE TABLE `rag_snippet` (
+  `rag_item_id` bigint NOT NULL COMMENT 'RAG item id (stable hash of table/version/snippet_id)',
+  `workspace_id` bigint NOT NULL COMMENT 'RAG workspace scope',
+  `table_id` bigint NOT NULL COMMENT '来源表ID',
+  `version_ts` bigint NOT NULL COMMENT '表版本号',
+  `action_result_id` bigint NOT NULL COMMENT '来源 action_results 主键ID（snippet_alias 或 snippet 行）',
+  `snippet_id` varchar(255) COLLATE utf8mb4_bin NOT NULL COMMENT '原始 snippet id',
+  `rag_text` text COLLATE utf8mb4_bin NOT NULL COMMENT '用于向量化的拼接文本',
+  `merged_json` json NOT NULL COMMENT '合并后的 snippet 对象',
+  `updated_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  PRIMARY KEY (`rag_item_id`),
+  KEY `idx_action_result` (`action_result_id`),
+  KEY `idx_workspace` (`workspace_id`),
+  KEY `idx_table_version` (`table_id`,`version_ts`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin COMMENT='RAG snippet 索引缓存';