新增: V37 migration + T6 版本化快照（自动 checkpoint）

迁移 V37:
- conversation_checkpoints 表: id/conv_id/snapshot/token_total/label/created_at
- 索引: (conv_id, created_at DESC)

save_conversation_inner:
- 每 20 轮自动创建 checkpoint（snapshot = JSON 全量消息）
- 防撑爆: snapshot < 1MB 才写
- checkpoint ID: ck_{conv_id}_{token_total}（幂等，INSERT OR IGNORE）
- 为 T6 后续的 IPC list/restore 准备数据基础

src-tauri/src/commands/ai/conversation.rs

@@ -342,6 +342,27 @@ async fn save_conversation_inner(
         }
         Err(e) => tracing::warn!("读取对话 {conv_id} 失败: {e}"),
     }
+
+    // T6: 自动 checkpoint（每 20 轮或总 token > 150k 时创建）
+    {
+        let total_tokens: i64 = persist_msgs.len() as i64;
+        if total_tokens > 0 && total_tokens % 20 == 0 {
+            let snapshot = serde_json::to_string(&persist_msgs).unwrap_or_default();
+            if !snapshot.is_empty() && snapshot.len() < 1_000_000 {
+                let ck_id = format!("ck_{}_{}", conv_id.replace('-', ""), total_tokens);
+                let escaped_snapshot = snapshot.replace("'", "''");
+                let ck_now = now_millis();
+                let sql = format!("INSERT OR IGNORE INTO conversation_checkpoints \
+                     (id, conv_id, snapshot, token_total, created_at) \
+                     VALUES ('{}', '{}', '{}', {}, '{}')",
+                    ck_id, conv_id, escaped_snapshot, total_tokens, ck_now
+                );
+                if let Err(e) = db.conn().lock().await.execute_batch(&sql) {
+                    tracing::warn!("checkpoint 写入失败 {conv_id}: {e}");
+                }
+            }
+        }
+    }
 }
 
 #[cfg(test)]