sync update from LAN repo (source dhk-1 @ 3b1ec5b4)

offline-backend-20260512/backend/docs/summarization.md

@@ -2,6 +2,25 @@
 
 DeerFlow includes automatic conversation summarization to handle long conversations that approach model token limits. When enabled, the system automatically condenses older messages while preserving recent context.
 
+## Administrator recent-three-turn window
+
+Administrators can enable a simpler deployment-wide sliding window in
+「设置 → 上下文管理」. It is stored as
+`system_settings.json -> conversation_context.recent_turn_window_enabled`, is
+disabled by default, and is exposed through
+`GET/PUT /api/system-settings/conversation-context` (write access is admin-only).
+
+When enabled, every lead-agent model call receives only the latest three real
+user turns, counting the current request, plus the assistant messages and tool
+results that belong to those turns. Hidden business context immediately attached
+to the first retained turn and system instructions are preserved. Trimming is
+performed on `ModelRequest` only, so checkpoint history and the transcript shown
+to the user remain complete.
+
+This mode takes priority over the per-chat 「上下文压缩」 option and does not run
+the summarization model. Turning it off restores the existing summarization
+behavior without losing earlier messages.
+
 ## Hard input limit (always-on safety net)
 
 Summarization is a quality feature and may be disabled by the user.  A model can