fix: 大屏统计按新品牌实体，材料种类统计改为细分种类并排除空值

- 材料种类卡片：从 material_subcategory 改为 material_category 去重计数，排除 null/空
- 材料子类分布图：排除 material_subcategory 为空的材料，避免出现 null 类目
- 品牌数卡片：从 Factory.count 改为 Brand.count
- 品牌材料分布图：按 material.brand.name 分组，排除无品牌材料；前端字段同步

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/apps/statistics/views.py

@@ -4,6 +4,7 @@ from rest_framework.response import Response
 from django.db.models import Count
 from apps.material.models import Material
 from apps.factory.models import Factory
+from apps.brand.models import Brand
 
 
 def _build_brochure_url(request, brochure_field):
@@ -27,26 +28,42 @@ def overview_statistics(request):
     # 材料总数
     total_materials = approved_materials.count()
 
-    # 材料种类（材料子类数量）
-    total_material_categories = approved_materials.values('material_subcategory').distinct().count()
+    # 材料种类（细分种类数量，排除空值）
+    total_material_categories = (
+        approved_materials
+        .exclude(material_category__isnull=True)
+        .exclude(material_category='')
+        .values('material_category')
+        .distinct()
+        .count()
+    )
 
-    # 品牌数（工厂数）
-    total_brands = Factory.objects.count()
+    # 品牌数
+    total_brands = Brand.objects.count()
 
     # 按专业类别的材料数量分布
     major_category_stats = approved_materials.values('major_category').annotate(
         count=Count('id')
     ).order_by('-count')
 
-    # 按材料子类的材料数量分布
-    material_subcategory_stats = approved_materials.values('material_subcategory').annotate(
-        count=Count('id')
-    ).order_by('-count')[:10]
+    # 按材料子类的材料数量分布（排除空值）
+    material_subcategory_stats = (
+        approved_materials
+        .exclude(material_subcategory__isnull=True)
+        .exclude(material_subcategory='')
+        .values('material_subcategory')
+        .annotate(count=Count('id'))
+        .order_by('-count')[:10]
+    )
 
-    # 按所属品牌的材料数量分布
-    brand_stats = approved_materials.values('factory__factory_name').annotate(
-        count=Count('id')
-    ).order_by('-count')
+    # 按所属品牌的材料数量分布（排除无品牌的材料）
+    brand_stats = (
+        approved_materials
+        .exclude(brand__isnull=True)
+        .values('brand__name')
+        .annotate(count=Count('id'))
+        .order_by('-count')
+    )
 
     # 按省份的工厂数量分布
     region_stats = Factory.objects.values('province').annotate(

frontend/src/views/screen/ScreenOverview.vue

@@ -113,7 +113,7 @@ const updateCharts = () => {
   })
 
   const brandData = (stats.value.brand_stats || []).map((item) => ({
-    name: item.factory__factory_name,
+    name: item.brand__name,
     value: item.count
   }))
   charts[2].setOption({