feat: 采购申请字段级价格权限 — 库管看不到采购价格

## permission_service.py
- init_all_menus: 新增3个采购权限元素
  inbound_purchase:unit_price, :total_price, :tax_rate

## purchase.py
- _filter_purchase_prices(): 无价格权限则pop价格字段
- GET /purchase (列表): 应用价格过滤
- GET /purchase/<id> (详情): 应用价格过滤
- approved-unstocked: 保留价格(入库数据源,前端按inbound_buy权限控制)

## purchase/index.vue
- 列表: 单价/总价/税率列 v-if hasPermission
- 详情: 单价/总价 v-if hasPermission

inventory-backend/app/services/permission_service.py

@@ -638,19 +638,23 @@ class PermissionService:
                         db.session.add(new_perm)
 
             db.session.commit()
-            # ★ 采购申请操作权限元素
-            purchase_op = SysElement.query.filter_by(
-                menu_code='inbound_purchase',
-                code='inbound_purchase:operation'
-            ).first()
-            if not purchase_op:
-                db.session.add(SysElement(
-                    menu_code='inbound_purchase',
-                    name='可编辑',
-                    code='inbound_purchase:operation',
-                    element_type='operation'
-                ))
-                print(f"✅ 采购申请操作权限元素已创建")
+            # ★ 采购申请权限元素
+            purchase_elements = [
+                ('inbound_purchase:operation', '可编辑', 'operation'),
+                ('inbound_purchase:unit_price', '采购单价', 'column'),
+                ('inbound_purchase:total_price', '采购总价', 'column'),
+                ('inbound_purchase:tax_rate', '税率', 'column'),
+            ]
+            for code, name, etype in purchase_elements:
+                existing = SysElement.query.filter_by(
+                    menu_code='inbound_purchase', code=code
+                ).first()
+                if not existing:
+                    db.session.add(SysElement(
+                        menu_code='inbound_purchase', name=name,
+                        code=code, element_type=etype
+                    ))
+                    print(f"✅ 采购申请元素已创建: {code}")
 
             print(f"✅ 所有菜单初始化完成")
             return True