fix(cma): scope ai-suggestion action_plan and preview KPI to session entity

_apply_action_plan: constrain the target KPIDefinition lookup to sug.entity_id so a
suggestion whose target_id/suggestion_data.kpi_id points at a foreign-entity KPI now
returns 404 instead of creating an ActionPlan row with a cross-entity kpi_id.

preview_suggestion: same constraint; a foreign-entity KPI now degrades to the existing
placeholder (KPI#<id>, current_target=None) instead of echoing the foreign KPI name and
current_target. Degradation (not 404) keeps the read-side preview usable when the stored
kpi_id is inconsistent.

Regression: backend/tests/test_ai_suggestion_kpi_attribution_residual_20260918.py
(RED 2 failed before, GREEN 5 passed after; full suite 1228+2F -> 1230 passed / 1 xfailed).

backend/app/api/ai_suggestions.py

@@ -236,7 +236,10 @@ def _apply_action_plan(db: Session, sug: AISuggestion, params: dict, current_use
     kpi_id = params.get("kpi_id") or sug.target_id
     if not kpi_id:
         raise HTTPException(400, "缺少 kpi_id")
-    kpi = db.query(KPIDefinition).filter(KPIDefinition.id == kpi_id).first()
+    kpi = db.query(KPIDefinition).filter(
+        KPIDefinition.id == kpi_id,
+        KPIDefinition.entity_id == sug.entity_id,   # 2026-09-18 派生单 2030b: 目标KPI须归属建议账套
+    ).first()
     if not kpi:
         raise HTTPException(404, f"KPI {kpi_id} 不存在")
 
@@ -311,7 +314,11 @@ def preview_suggestion(suggestion_id: int, db: Session = Depends(get_db),
     kpi = None
     kpi_id = sd.get("kpi_id") or sug.target_id
     if kpi_id:
-        kpi = db.query(KPIDefinition).filter(KPIDefinition.id == kpi_id).first()
+        # 2026-09-18 派生单 2030b: 目标KPI须归属建议账套；对端KPI降级为 KPI#<id>（不回显名/目标值）
+        kpi = db.query(KPIDefinition).filter(
+            KPIDefinition.id == kpi_id,
+            KPIDefinition.entity_id == sug.entity_id,
+        ).first()
 
     if sug.suggestion_type == "kpi_target":
         return {"data": {