feat: 新增备料（采购下单）功能模块，含清空清单

- 独立采购备料清单：选 BOM + 输入套数一键生成计划（计划数量 = 单套用量 × 套数），可手工增删改、导出 Excel

- 备料与生产订单完全解耦，仅记录计划数量、不记账（不产生任何出入库流水）

- 新增「清空清单」功能（/prepare/clear，JS 二次确认 + 操作日志），按当前工厂范围清空

- 配套：PrepPlan 模型、缺口(计划-库存)实时计算、库存随工厂切换过滤

excel_io.py

@@ -245,6 +245,35 @@ def export_inventory(path, rows):
     wb.save(path)
 
 
+def export_prep(path, rows, scope=""):
+    """导出备料清单(采购下单用, 不记账; 手工维护, 与生产订单无关)。
+
+    rows: [{id, m, plan(计划数量), stock(实际库存), diff(缺口=计划-库存), short}]
+    scope: 范围说明(工厂/全部)。
+    """
+    wb = Workbook()
+    ws = wb.active
+    ws.title = "备料清单"
+    head = ["物料编号", "型号/规格", "单位", "计划数量", "实际库存",
+            "缺口(计划-库存)", "状态", "范围"]
+    ws.append(head)
+    for c in ws[1]:
+        c.font = Font(bold=True)
+        c.fill = PatternFill("solid", fgColor="DDEBF7")
+    for r in rows:
+        m = r["m"]
+        status = "需采购" if r["diff"] > 0 else ("刚好够" if r["diff"] == 0 else "已有盈余")
+        ws.append([m.part_number, m.manufacturer or "", m.unit or "个",
+                   r["plan"], r["stock"], r["diff"], status, scope])
+        # 缺口着色: >0 需采购标红; <0 盈余标绿
+        if r["diff"] > 0:
+            ws.cell(row=ws.max_row, column=6).fill = PatternFill("solid", fgColor="FFC7CE")
+        elif r["diff"] < 0:
+            ws.cell(row=ws.max_row, column=6).fill = PatternFill("solid", fgColor="C6EFCE")
+    ws.freeze_panes = "A2"
+    wb.save(path)
+
+
 def build_material_template(path):
     wb = Workbook()
     ws = wb.active