fix: 预算流程断点修复 — 现金流联动+行动方案补齐

断点#1: 预算↔现金流打通
- cash_plans 加 related_kpi_id/budget_plan_id 字段
- POST /budget/sync-cash-plans: 按预算KPI生成收付款计划(收入类→receive/成本类→pay, upsert)
- 前端预算执行页加'⇄同步现金流计划'按钮
- 实测: 新建108条(receive 75/pay 33, 6个KPI)

断点#2: 5个预算KPI补行动方案(净利/新客/厂补/供应链/数据自动化, 各1条含负责人)

断点#3: 悬空预算(2KPI)因已补行动方案, 性质从'无来源'→'有行动缺实际数据源', 标注保留

pytest 56 passed(budget+cash), 构建部署

backend/app/models/__init__.py

@@ -485,6 +485,8 @@ class CashPlan(Base):
     id = Column(Integer, primary_key=True, index=True)
     entity_id = Column(Integer, ForeignKey("entities.id"), default=1, comment="企业ID")
     plan_type = Column(String(10), nullable=False, comment="receive收/pay付")
+    related_kpi_id = Column(Integer, nullable=True, comment="关联KPI(预算联动 2026-08-27)")
+    budget_plan_id = Column(Integer, nullable=True, comment="来源预算计划ID")
     amount = Column(Float, nullable=False, comment="金额(万元)")
     plan_date = Column(DateTime, nullable=False, comment="计划日期(应收即到期日)")
     counterparty = Column(String(200), nullable=True, comment="关联客户/供应商")