feat(conversations,api,ui): rewind takes the last turns of a thread back, the cut session stays in the store

src/beaver_gateway/conversations/turns.py

@@ -68,14 +68,14 @@ class Turns(Seeds):
         backend = self._backend(conv.agent_name)
         turn_id = turn_id or f"turn_{uuid4().hex[:12]}"
         capture = capture or TurnCapture()
-        resume = session_id if session_id is not None else conv.session_id
         async with runner.lock:
             runner.turn_id = turn_id
             runner.origin = origin
             runner.text = shown if shown is not None else _prompt_preview(messages)
             runner.started_at = datetime.now(UTC)
             runner.tools = {}
-            await self._mark_running(conv, turn_id)
+            row = await self._mark_running(conv, turn_id)
+            resume = session_id if session_id is not None else row.session_id
             before = await self.before_turn(conv)
             self._bus.publish(
                 "turn.start",
@@ -463,12 +463,12 @@ class Turns(Seeds):
         except Exception:  # noqa: BLE001
             _log.exception("rate limit write failed")
 
-    async def _mark_running(self, conv: Conversation, turn_id: str) -> None:
+    async def _mark_running(self, conv: Conversation, turn_id: str) -> Conversation:
         async def apply(row: Conversation) -> None:
             row.running_turn = turn_id
             row.last_activity_at = datetime.now(UTC)
 
-        await self._update(conv, apply)
+        return await self._update(conv, apply)
 
     async def _mark_done(
         self, conv: Conversation, capture: TurnCapture, *, cut: bool = False