fix(gateway): 压缩期间释放 session 锁，修复新建话题列表刷新延迟

压缩任务在 LLM 调用（2-5s）期间持有 session 锁，阻塞 create_session
命令获取锁执行 switch_topic，导致新建话题后列表刷新延迟。

将压缩重构为 4 阶段：
1. 短暂持锁：读取 history/compressor/store/session_id/provider_config
2. 释放锁：LLM 压缩调用（2-5s）
3. 释放锁：DB 写入（store 为 Arc，DB 层自带事务保护）
4. 短暂持锁：reload 内存历史

并发安全：同 topic 由调用方的 per-topic serial lock 保证串行；
不同 topic 完全并行不受影响。

src/gateway/compaction.rs

@@ -6,13 +6,20 @@ use crate::agent::AgentError;
 
 use super::session::Session;
 
-/// Run two-segment history compression synchronously.
+/// Run two-segment history compression.
 ///
-/// Unlike the previous background approach (tokio::spawn), this holds the
-/// session lock during the LLM calls (2–5 seconds).  Since the agent loop
-/// has already finished by this point there is no response-time impact, and
-/// the synchronous guarantee means the next execution always starts with
-/// freshly compacted history.
+/// The session lock is held only for the brief data-gathering and
+/// history-reload phases. The expensive LLM call (2–5 seconds) and the DB
+/// write happen **without** holding the session lock, so other commands
+/// (e.g. `create_session`, `list_topics`) are not blocked during compression.
+///
+/// Concurrency safety:
+/// - **Same topic**: the caller (`prepare_and_execute_message`) holds the
+///   per-topic serial lock (`_serial_guard`) for the entire duration of
+///   execution + compaction, so no other message for this topic can modify
+///   the in-memory or DB history between phases.
+/// - **Different topic**: fully unblocked — the session lock is free during
+///   the LLM call.
 ///
 /// 按 topic_id 隔离：压缩只处理指定 topic 的历史，DB 替换也只影响该 topic。
 pub(crate) async fn schedule_background_history_compaction(
@@ -23,35 +30,41 @@ pub(crate) async fn schedule_background_history_compaction(
     let chat_id = chat_id.into();
     let topic_id = topic_id.into();
 
+    // Phase 1: brief session lock to gather compaction inputs.
+    let (history, compressor, store, session_id, provider_config) = {
         let mut session_guard = session.lock().await;
         session_guard.ensure_persistent_session(&chat_id)?;
         session_guard.ensure_chat_loaded(&chat_id, Some(&topic_id))?;
 
         let history = session_guard.get_or_create_history(&topic_id).clone();
         let compressor = session_guard.compressor().clone();
+        let store = session_guard.store();
+        let session_id = session_guard.persistent_session_id(&chat_id);
+        let provider_config = session_guard.provider_config().clone();
+        (history, compressor, store, session_id, provider_config)
+    };
+    // session lock released here
 
     if !compressor.should_compress(&history) {
         return Ok(());
     }
 
-    let store = session_guard.store();
-    let session_id = session_guard.persistent_session_id(&chat_id);
-    let provider_config = session_guard.provider_config().clone();
-
     tracing::info!(
         chat_id = %chat_id,
         topic_id = %topic_id,
         msg_count = history.len(),
-        "Starting synchronous two-segment compression"
+        "Starting two-segment compression (session lock released during LLM call)"
     );
 
-    // Synchronous compression — holds lock during LLM calls.
+    // Phase 2: LLM compression WITHOUT holding the session lock.
     // compress_two_segment guarantees the result contains no tool_calls,
     // so there is no risk of orphaned tool call sequences.
     let compressed = compressor
         .compress_two_segment(&history, &provider_config)
         .await?;
 
+    // Phase 3: DB write — store is Arc<dyn ConversationRepository>, no
+    // session lock needed.
     // 保留原始消息（标记 is_compacted=1）+ 插入压缩摘要，不删除原消息，
     // 从而让前端仍能展示完整原始对话，LLM 只看压缩后的精简历史。
     store
@@ -65,7 +78,11 @@ pub(crate) async fn schedule_background_history_compaction(
         "Two-segment compression committed (original messages retained)"
     );
 
+    // Phase 4: re-acquire session lock to refresh in-memory history.
+    {
+        let mut session_guard = session.lock().await;
         session_guard.reload_topic_history(&chat_id, &topic_id)?;
+    }
 
     Ok(())
 }