Guard the news detail panel against stale responses and load errors

The detail is now a persistent side panel, so quick clicks can interleave
requests: only the latest request's result is applied, and a failed load
shows an error state with a retry button instead of spinning forever.

Also correct the position history comments: only plans from before plan
3.1 could hand their position over to a replacing plan.

backend/src/alphapulse/api/trading.py

@@ -149,7 +149,7 @@ class PositionHistoryOut(BaseModel):
     opened_at: datetime
     closed_at: datetime
     close_reason: str
-    # 本计划的入场成交；计划被替换时持仓由新计划接管，新计划没有自己的入场成交，这两项为空
+    # 本计划的入场成交。计划 3.1 之前，替换计划时持仓会交给新计划接管（如 #8），这类旧记录没有自己的入场成交，这两项为空
     quantity: Decimal | None
     entry_price: Decimal | None
     # 止盈、止损、市价平仓的成交均价；外部平仓（如网页手动平仓）没有成交记录，为空