fix(v2): 退款自愈扫描加回溯窗(防 LIMIT 尾部饥饿)+ 清死行

RefundApplyTask 的候选查询按 out_trade_no ASC/id ASC 取前 200,succeeded 退款历史
只增不减、partially_refunded 长期驻留,超过 limit 后最新的崩溃窗口永远排在候选外
扫不到。两路查询都加 since 回溯窗:RefundStore.ListDistinctOutTradeNosByStatusSince
(completed_at>=since)、OrderStore.ListOrdersByStatusSince(updated_at>=since),旧的
无窗方法原样保留。RefundApplyTask 新增 lookback/now 参数,config.ReconcileConfig 新增
RefundApplyLookbackMin(默认 48h)。顺带按 T6 复审补 OrderTTLMin 字段注释(须 ≫ crypto
orderTTL 15min),清 sync_test.go 死行 `_ = time.Second`。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

internal/store/refund_test.go

@@ -126,6 +126,29 @@ func TestListDistinctOutTradeNosByStatus(t *testing.T) {
 	}
 }
 
+// TestListDistinctOutTradeNosByStatusSinceExcludesStale 钉住回溯窗:completed_at
+// 早于 since 的 succeeded 退款不应入候选(防 LIMIT 尾部饥饿——见 RefundApplyTask)。
+func TestListDistinctOutTradeNosByStatusSinceExcludesStale(t *testing.T) {
+	db := model.OpenTestDB(t)
+	rs := NewRefundStore(db)
+	now := time.Now()
+	stale := now.Add(-72 * time.Hour)
+	fresh := now.Add(-time.Hour)
+
+	_ = rs.CreateRefund(&model.Refund{RefundID: "s-stale", OutTradeNo: "PAY-STALE", AmountMinor: 100, Currency: "CNY",
+		Status: model.RefundSucceeded, CompletedAt: &stale})
+	_ = rs.CreateRefund(&model.Refund{RefundID: "s-fresh", OutTradeNo: "PAY-FRESH", AmountMinor: 100, Currency: "CNY",
+		Status: model.RefundSucceeded, CompletedAt: &fresh})
+
+	got, err := rs.ListDistinctOutTradeNosByStatusSince(model.RefundSucceeded, now.Add(-48*time.Hour), 50)
+	if err != nil {
+		t.Fatalf("list: %v", err)
+	}
+	if len(got) != 1 || got[0] != "PAY-FRESH" {
+		t.Fatalf("只应命中窗内的 PAY-FRESH, got %+v", got)
+	}
+}
+
 // TestListStuckRefunds 供「卡滞 processing/manual_pending 退款告警」:只挑 updated_at
 // 早于阈值的 processing/manual_pending 行,requested/succeeded/failed 不命中。
 func TestListStuckRefunds(t *testing.T) {