refactor(material): 改用显式失效缓存，移除 signals；TTL 30min → 5min

- 删除 signals.py，改在 approve / reject / perform_destroy / import_excel 四个会影响
  已审核材料集合的入口显式调用 invalidate_category_tree_cache()，调用栈可见、易追踪
- reject 与 perform_destroy 仅当原状态为 approved 时才失效，避免无效缓存抖动
- TTL 由 30 分钟降为 5 分钟，作为兜底防止遗漏路径

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

backend/apps/material/apps.py

@@ -4,6 +4,3 @@ from django.apps import AppConfig
 class MaterialConfig(AppConfig):
     default_auto_field = 'django.db.models.BigAutoField'
     name = 'apps.material'
-
-    def ready(self):
-        from . import signals  # noqa: F401

backend/apps/material/signals.py

@@ -1,21 +0,0 @@
-from django.core.cache import cache
-from django.db.models.signals import post_delete, post_save
-from django.dispatch import receiver
-
-from .models import Material
-
-CATEGORY_TREE_CACHE_KEY = 'material:category_tree:approved'
-
-
-def invalidate_category_tree_cache():
-    cache.delete(CATEGORY_TREE_CACHE_KEY)
-
-
-@receiver(post_save, sender=Material)
-def _on_material_saved(sender, instance, **kwargs):
-    invalidate_category_tree_cache()
-
-
-@receiver(post_delete, sender=Material)
-def _on_material_deleted(sender, instance, **kwargs):
-    invalidate_category_tree_cache()

backend/apps/material/views.py

@@ -19,10 +19,16 @@ from django.core.cache import cache
 from .models import Material, MaterialCategory, MaterialSubcategory
 from .serializers import MaterialSerializer, MaterialListSerializer, MaterialCategorySerializer, MaterialSubcategorySerializer
 from .importers import import_materials_plan_excel
-from .signals import CATEGORY_TREE_CACHE_KEY
 from apps.factory.models import COOPERATION_MODE_CHOICES
 
 
+CATEGORY_TREE_CACHE_KEY = 'material:category_tree:approved'
+
+
+def invalidate_category_tree_cache():
+    cache.delete(CATEGORY_TREE_CACHE_KEY)
+
+
 def _join_choice_values(values, choices):
     if not values:
         return ""
@@ -296,7 +302,10 @@ class MaterialViewSet(ModelViewSet):
         # 普通用户只能删除创建中的材料
         if self.request.user.role != 'admin' and instance.status != 'draft':
             raise PermissionDenied("只有创建中的材料可以删除")
+        was_approved = instance.status == 'approved'
         instance.delete()
+        if was_approved:
+            invalidate_category_tree_cache()
 
     @action(detail=True, methods=['post'])
     def submit(self, request, pk=None):
@@ -344,6 +353,7 @@ class MaterialViewSet(ModelViewSet):
 
         material.status = 'approved'
         material.save()
+        invalidate_category_tree_cache()
         return Response({"status": "审核通过"})
 
     @action(detail=True, methods=['post'])
@@ -365,8 +375,11 @@ class MaterialViewSet(ModelViewSet):
                 status=status.HTTP_400_BAD_REQUEST
             )
 
+        was_approved = material.status == 'approved'
         material.status = 'draft'
         material.save()
+        if was_approved:
+            invalidate_category_tree_cache()
         return Response({"status": "审核拒绝"})
 
     @action(detail=False, methods=['get'])
@@ -484,6 +497,7 @@ class MaterialViewSet(ModelViewSet):
         except Exception as exc:
             return Response({"detail": f"导入失败: {exc}"}, status=status.HTTP_400_BAD_REQUEST)
 
+        invalidate_category_tree_cache()
         return Response(result)
 
     @action(detail=False, methods=['get'], url_path='category-tree', permission_classes=[IsAuthenticated])
@@ -536,7 +550,7 @@ class MaterialViewSet(ModelViewSet):
                 ],
             })
 
-        cache.set(CATEGORY_TREE_CACHE_KEY, data, timeout=60 * 30)
+        cache.set(CATEGORY_TREE_CACHE_KEY, data, timeout=60 * 5)
         return Response(data)
 
     @action(detail=False, methods=['get'], url_path='categories-by-major')