feat(backend): 组织序列化器添加在招岗位数统计

- JobListSerializer 和 JobDetailSerializer 新增 job_count 字段
- 统计 status='published' 的岗位数量
- 前端展示每个公司/子公司的在招岗位数

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

offer_backend/apps/organizations/serializers.py

@@ -3,18 +3,27 @@ from .models import Organization
 
 
 class OrganizationSerializer(serializers.ModelSerializer):
+    job_count = serializers.SerializerMethodField()
+
     class Meta:
         model = Organization
-        fields = ['id', 'name', 'parent', 'logo', 'description', 'email', 'is_active']
+        fields = ['id', 'name', 'parent', 'logo', 'description', 'email', 'is_active', 'job_count']
+
+    def get_job_count(self, obj):
+        return obj.jobs.filter(status='published').count()
 
 
 class OrganizationTreeSerializer(serializers.ModelSerializer):
     """带子公司列表，用于门户展示"""
     children = serializers.SerializerMethodField()
+    job_count = serializers.SerializerMethodField()
 
     class Meta:
         model = Organization
-        fields = ['id', 'name', 'logo', 'description', 'email', 'children']
+        fields = ['id', 'name', 'logo', 'description', 'email', 'job_count', 'children']
+
+    def get_job_count(self, obj):
+        return obj.jobs.filter(status='published').count()
 
     def get_children(self, obj):
         return OrganizationSerializer(