feat(P1-3): 相关成本决策模块 — 官方25%权重'商业决策分析'核心

新增 /predict/relevant-decision API + 前端Tab(3场景):
1. 自制vs外购: 相关成本比较+无差别点(增量成本视角)
2. 特殊订单: 剩余产能下价格>变动成本即接受(增量利润)
3. 产品组合(约束理论): 单位约束资源边际贡献排序

验证: 自制850k vs 外购900k→自制; 特殊订单单位贡献25→接受;
      产品组合B产品单位约束贡献50最高→优先

backend/app/api/predict.py

@@ -161,6 +161,97 @@ def api_cvp_detailed(data: dict):
         raise HTTPException(400, f"CVP详细分析失败: {str(e)}")
 
 
+# ── 相关成本决策（CMA P2商业决策分析25%权重核心） ──────────────────
+
+
+@router.post("/relevant-decision")
+def api_relevant_decision(data: dict):
+    """相关成本决策分析（CMA P2 商业决策分析核心内容）
+    场景: make-or-buy自制外购 / special-order特殊订单 / product-mix产品组合
+    """
+    try:
+        decision_type = data.get("type", "make_or_buy")
+
+        if decision_type == "make_or_buy":
+            # 自制vs外购决策
+            # 相关成本 = 增量成本（只有随决策变化的成本才是相关的）
+            make_var_cost = float(data.get("make_variable_cost", 0))   # 自制单位变动成本
+            make_fixed = float(data.get("make_fixed_cost", 0))         # 自制新增固定成本
+            buy_price = float(data.get("buy_price", 0))                # 外购单价
+            demand = float(data.get("demand", 0))                      # 需求量
+            existing_fixed = float(data.get("existing_fixed_cost", 0)) # 现有固定成本（无关成本，自制不增加则忽略）
+
+            make_total = make_var_cost * demand + make_fixed
+            buy_total = buy_price * demand
+            diff = buy_total - make_total  # >0自制省钱
+
+            return {
+                "type": "自制vs外购",
+                "make_total_cost": round(make_total, 2),
+                "buy_total_cost": round(buy_total, 2),
+                "difference": round(diff, 2),
+                "recommendation": "自制" if diff > 0 else "外购",
+                "reason": f"自制总成本{make_total:.2f} vs 外购总成本{buy_total:.2f}，{'自制节省' + str(round(diff,2)) if diff > 0 else '外购节省' + str(round(-diff,2))}",
+                "unit_make_cost": round(make_var_cost + (make_fixed / demand if demand else 0), 2),
+                "unit_buy_price": buy_price,
+                "indifferent_point": round(make_fixed / (buy_price - make_var_cost), 2) if buy_price > make_var_cost else None,
+                "notes": "仅考虑相关成本(增量成本)；现有固定成本若不受决策影响则无关",
+            }
+
+        elif decision_type == "special_order":
+            # 特殊订单决策（有剩余产能时，只要价格>单位变动成本即接受）
+            normal_price = float(data.get("normal_price", 0))
+            special_price = float(data.get("special_price", 0))
+            var_cost = float(data.get("variable_cost", 0))
+            order_qty = float(data.get("order_qty", 0))
+            capacity_used = float(data.get("capacity_used", 0))  # 特殊订单占用产能%
+            extra_fixed = float(data.get("extra_fixed_cost", 0))  # 一次性额外固定成本
+
+            contribution_per_unit = special_price - var_cost
+            total_contribution = contribution_per_unit * order_qty - extra_fixed
+            accept = total_contribution > 0 and capacity_used <= 100
+
+            return {
+                "type": "特殊订单",
+                "unit_contribution": round(contribution_per_unit, 2),
+                "total_contribution": round(total_contribution, 2),
+                "extra_fixed_cost": extra_fixed,
+                "capacity_used_pct": capacity_used,
+                "recommendation": "接受" if accept else "拒绝",
+                "reason": f"单价{special_price} - 变动成本{var_cost} = 单位贡献{contribution_per_unit:.2f}" + 
+                          (f"，共{total_contribution:.2f} > 0 且产能{capacity_used}%够用 → 接受(增量利润)" if accept else 
+                           f"，总贡献{total_contribution:.2f} ≤ 0 或产能不足 → 拒绝"),
+                "notes": "有剩余产能时，只要价格>变动成本且不冲击正常市场即可接受；固定成本无关",
+            }
+
+        elif decision_type == "product_mix":
+            # 产品组合决策（约束理论：单位约束资源的边际贡献最大者优先）
+            products = data.get("products", [])  # [{name, price, var_cost, constraint_usage, demand}]
+            results = []
+            for p in products:
+                cm_per_unit = float(p.get("price", 0)) - float(p.get("var_cost", 0))
+                cm_per_constraint = cm_per_unit / float(p.get("constraint_usage", 1))
+                results.append({
+                    "name": p.get("name", ""),
+                    "unit_contribution": round(cm_per_unit, 2),
+                    "constraint_usage": float(p.get("constraint_usage", 1)),
+                    "contribution_per_constraint": round(cm_per_constraint, 2),
+                    "demand": float(p.get("demand", 0)),
+                })
+            # 按单位约束资源贡献排序（约束理论优先）
+            results.sort(key=lambda x: x["contribution_per_constraint"], reverse=True)
+            return {
+                "type": "产品组合(约束理论)",
+                "ranking": results,
+                "recommendation": f"优先生产「{results[0]['name']}」(单位约束贡献{results[0]['contribution_per_constraint']}最高)",
+                "notes": "瓶颈资源下，按单位约束资源的边际贡献排序，而非单位边际贡献",
+            }
+
+        raise HTTPException(400, "未知决策类型: " + str(decision_type))
+    except Exception as e:
+        raise HTTPException(400, f"相关成本决策失败: {str(e)}")
+
+
 # ── 现金流预测（AI事前预警） ────────────────────────────────────