fix(cma): entity isolation batch3 (cost abc write endpoints + allocations/stats reads + plan kr_id scope)

- cost.py: POST /abc/activities 按会话账套写入 entity_id；POST /abc/allocate 校验 activity_id 归属本账套(跨账套 404)并透传 entity_id；GET /abc/allocations 加账套过滤
- cost_engine.py: allocate_cost(entity_id=None) 按账套过滤作业中心 + 显式写入 AbcAllocation.entity_id（不传维持原语义）
- action_plans.py: update_plan 的 kr_id 加账套校验(跨账套 404，对齐 create_plan 既有写法)；plan_stats join KPIDefinition 按账套过滤(口径与 list_plans 一致)
- tests: 新增 tests/test_cross_entity_isolation_batch3_20260918.py（10 例；改前 7 failed/3 passed → 改后 10 passed）
- 回归: 1184 passed,1 xfailed → 1194 passed,1 xfailed（无新增失败）

backend/app/utils/cost_engine.py

@@ -158,11 +158,21 @@ def calc_driver_rate(activity_id: int) -> dict:
 
 
 def allocate_cost(activity_id: int, period: str, product_code: str,
-                  product_name: str, driver_consumed: float) -> dict:
-    """按动因分配成本到产品"""
+                  product_name: str, driver_consumed: float,
+                  entity_id: Optional[int] = None) -> dict:
+    """按动因分配成本到产品
+
+    账套隔离（2026-09-18，project-20260918-entity-isolation-batch3）：
+      - entity_id 非 None 时，作业中心查询按本账套过滤（跨账套 activity_id 查不到 → 返回错误字典，
+        端点侧已先行 404）；
+      - 新建 AbcAllocation 显式写入 entity_id（不传则维持原语义：不写归属，落模型默认）。
+    """
     db = get_session_local()()
     try:
-        act = db.query(AbcActivity).filter(AbcActivity.id == activity_id).first()
+        q_act = db.query(AbcActivity).filter(AbcActivity.id == activity_id)
+        if entity_id is not None:
+            q_act = q_act.filter(AbcActivity.entity_id == entity_id)
+        act = q_act.first()
         if not act or act.driver_rate == 0:
             # 自动计算分配率
             if act and act.driver_volume > 0:
@@ -171,7 +181,7 @@ def allocate_cost(activity_id: int, period: str, product_code: str,
         if not act or act.driver_rate == 0:
             return {"error": "分配率未设置"}
         allocated = round(driver_consumed * act.driver_rate, 2)
-        alloc = AbcAllocation(
+        alloc_kwargs = dict(
             period=period,
             activity_id=activity_id,
             product_code=product_code,
@@ -179,6 +189,9 @@ def allocate_cost(activity_id: int, period: str, product_code: str,
             driver_consumed=driver_consumed,
             allocated_cost=allocated,
         )
+        if entity_id is not None:
+            alloc_kwargs["entity_id"] = entity_id
+        alloc = AbcAllocation(**alloc_kwargs)
         db.add(alloc)
         db.commit()
         return {