fix: 应收预警重复生成Bug — 按plan_id去重替代全文匹配

根因: 到期未收款预警消息含'已逾期X天'动态文本, _exists()按全文匹配
  每天逾期天数变化→消息不同→去重永远失败→每天生成重复预警(68条)

修复: 到期未收款循环改为按plan_id查已有pending预警去重
验证: 连续触发2次均新增0条

附带修复:
- 清理159条金额单位错误预警(元→万元, 40980万→4.1万)
- 删除137条budget_sync虚拟收付款计划
- 作废129条孤儿预警(指向已删除plan)
- 清理68条重复预警

backend/app/utils/cash_forecast_engine.py

@@ -680,7 +680,21 @@ def check_cash_alerts(db: Session, entity_id: int = 1) -> dict:
             days_late = (today - p.plan_date).days
             msg = (f"【到期未收款】应收款{p.counterparty or '客户'} {p.amount:.1f}万 "
                    f"原计划{p.plan_date.strftime('%Y-%m-%d')}到期，已逾期{days_late}天未收回")
-            if _exists(msg):
+            # 2026-08-30修复: 按plan_id去重（消息含"已逾期X天"动态文本，全文匹配会每天生成重复预警）
+            existing_plan_alert = db.query(KPIAlert).filter(
+                KPIAlert.alert_type == "cash_plan",
+                KPIAlert.status.in_(["pending", "processing"]),
+            ).all()
+            dup = False
+            for ea in existing_plan_alert:
+                try:
+                    es = _json.loads(ea.suggestion) if ea.suggestion else {}
+                    if isinstance(es, dict) and es.get("plan_id") == p.id:
+                        dup = True
+                        break
+                except Exception:
+                    continue
+            if dup:
                 continue
             alert = KPIAlert(
                 kpi_id=ar_kpi.id,