新增: 消息分页懒加载与事件总线空转优化

后端基础设施两项改进:

一、历史消息分页懒加载(治长对话渲染卡顿)
- 消息仓库新增分页查询方法(从尾部取最近 N 条,支持游标)
- 切换对话时后端保留全量消息供模型上下文用,但只返回最近
  50 条给前端渲染,附带是否有更多历史和游标位置
- 新增加载更多消息的接口,前端滚顶时按游标分页拉取

二、事件总线空转优化(消除无消费者时的序列化开销)
- 发布事件前先检查订阅者数量,无订阅者直接跳过序列化
- 当前隧道订阅方未接入,20 余个发射点双写不再空转浪费

同时核验确认对话上下文透明化三项均已落地(目标可见/
项目增强预览/完整上下文面板),关闭对应待办。

crates/df-storage/src/crud/message_repo.rs

@@ -128,6 +128,57 @@ impl AiMessageRepo {
         .map_err(storage_err)?
     }
 
+    /// 按对话加载最近 N 条消息(分页懒加载,治长对话渲染卡顿)。
+    ///
+    /// 从尾部取最近 `limit` 条(ORDER BY seq DESC LIMIT),返回时反转为 ASC 顺序(与
+    /// list_by_conversation 一致的 seq 升序)。`before_seq` 可选:指定后只取 seq < before_seq
+    /// 的消息(滚顶加载更多时的游标,取下一页更早的历史)。
+    ///
+    /// 典型用法:
+    /// - 首次切入对话:list_recent(conv_id, 50, None) → 最近 50 条
+    /// - 滚顶加载更多:list_recent(conv_id, 50, Some(最早已加载消息的 seq)) → 再加载 50 条更早的
+    pub async fn list_recent(
+        &self,
+        conversation_id: &str,
+        limit: usize,
+        before_seq: Option<i64>,
+    ) -> Result<Vec<AiMessageRecord>> {
+        let conn = self.conn.clone();
+        let conv_id = conversation_id.to_owned();
+        tokio::task::spawn_blocking(move || {
+            let guard = conn.blocking_lock();
+            // before_seq 有无分两个 SQL(参数化 LIMIT 必须用固定占位,Rust 侧 clamp 防 0)
+            let limit = limit.max(1) as i64;
+            let sql = if before_seq.is_some() {
+                "SELECT id, conversation_id, seq, role, content, parts, tool_call_id,
+                        tool_calls, model, status, reasoning_content, timestamp, created_at
+                 FROM ai_messages WHERE conversation_id = ?1 AND seq < ?2 ORDER BY seq DESC LIMIT ?3"
+            } else {
+                "SELECT id, conversation_id, seq, role, content, parts, tool_call_id,
+                        tool_calls, model, status, reasoning_content, timestamp, created_at
+                 FROM ai_messages WHERE conversation_id = ?1 ORDER BY seq DESC LIMIT ?2"
+            };
+            let mut stmt = guard.prepare(sql).map_err(storage_err)?;
+            let rows = match before_seq {
+                Some(seq) => stmt
+                    .query_map(params![conv_id, seq, limit], ai_message_from_row)
+                    .map_err(storage_err)?,
+                None => stmt
+                    .query_map(params![conv_id, limit], ai_message_from_row)
+                    .map_err(storage_err)?,
+            };
+            let mut results: Vec<AiMessageRecord> = Vec::new();
+            for r in rows {
+                results.push(r.map_err(storage_err)?);
+            }
+            // DESC → 反转为 ASC(与 list_by_conversation 一致顺序)
+            results.reverse();
+            Ok(results)
+        })
+        .await
+        .map_err(storage_err)?
+    }
+
     /// 删除对话内 seq ∈ [min_seq, max_seq) 的消息(左闭右开)。
     ///
     /// compress 压缩 / 编辑重生成 dirty 范围重写用:delete_range → insert_batch 原子覆盖。