fix(knowledge): verification audit — 3 medium issues

- create_item: wrap item + version INSERT in transaction for atomicity
- update_item handler: validate content length (100KB) before DB hit
- KnowledgeChunk: document missing embedding field, safe per explicit SELECT usage

crates/zclaw-saas/src/knowledge/service.rs

@@ -296,6 +296,9 @@ pub async fn create_item(
         ));
     }
 
+    // 使用事务保证 item + version 原子性
+    let mut tx = pool.begin().await?;
+
     let item = sqlx::query_as::<_, KnowledgeItem>(
         "INSERT INTO knowledge_items \
          (id, category_id, title, content, keywords, related_questions, priority, tags, created_by) \
@@ -311,7 +314,7 @@ pub async fn create_item(
     .bind(priority)
     .bind(tags)
     .bind(account_id)
-    .fetch_one(pool)
+    .fetch_one(&mut *tx)
     .await?;
 
     // 创建初始版本快照
@@ -328,9 +331,10 @@ pub async fn create_item(
     .bind(keywords)
     .bind(related_questions)
     .bind(account_id)
-    .execute(pool)
+    .execute(&mut *tx)
     .await?;
 
+    tx.commit().await?;
     Ok(item)
 }