feat: 新增Snippet全局内容模型（TeamMember/Testimonial/Partner/NavigationMenu/SiteSettings）

- 新增 apps.core.TeamMember/Testimonial/Partner：简单 register_snippet，含 order 排序字段
- 新增 apps.core.NavigationMenu+NavigationMenuItem：ClusterableModel+Orderable+InlinePanel，
  支持内部页面或外部链接二选一
- 新增 apps.core.SiteSettings：接入 wagtail.contrib.settings + BaseSiteSetting，
  存放公司信息/ICP备案号/社交账号等全局配置
- 新增只读 API：/api/v1/custom/core/{team,testimonials,partners,navigation,site-settings}/
- 补充 apps/core/tests.py 单元测试，pytest 用例由 11 个增至 24 个，全部通过
- 更新设计文档 §2.16 Phase 1 进度清单

apps/core/serializers.py

@@ -0,0 +1,74 @@
+"""
+apps.core 全局 Snippet 的只读序列化器，供自定义业务接口
+（/api/v1/custom/core/...）使用。图片字段统一序列化为
+{"url": ..., "title": ...}，与前端 frontend/types/wagtail.ts
+中既有的 background_image/logos 字段结构保持一致。
+"""
+from rest_framework import serializers
+
+
+def serialize_image(image):
+    if not image:
+        return None
+    return {"url": image.file.url, "title": image.title}
+
+
+class TeamMemberSerializer(serializers.Serializer):
+    id = serializers.IntegerField()
+    name = serializers.CharField()
+    role = serializers.CharField()
+    bio = serializers.CharField()
+    photo = serializers.SerializerMethodField()
+
+    def get_photo(self, obj):
+        return serialize_image(obj.photo)
+
+
+class TestimonialSerializer(serializers.Serializer):
+    id = serializers.IntegerField()
+    quote = serializers.CharField()
+    author_name = serializers.CharField()
+    author_title = serializers.CharField()
+    author_photo = serializers.SerializerMethodField()
+
+    def get_author_photo(self, obj):
+        return serialize_image(obj.author_photo)
+
+
+class PartnerSerializer(serializers.Serializer):
+    id = serializers.IntegerField()
+    name = serializers.CharField()
+    website_url = serializers.CharField()
+    logo = serializers.SerializerMethodField()
+
+    def get_logo(self, obj):
+        return serialize_image(obj.logo)
+
+
+class NavigationMenuItemSerializer(serializers.Serializer):
+    label = serializers.CharField()
+    url = serializers.CharField()
+    open_in_new_tab = serializers.BooleanField()
+
+
+class NavigationMenuSerializer(serializers.Serializer):
+    name = serializers.CharField()
+    items = serializers.SerializerMethodField()
+
+    def get_items(self, obj):
+        return NavigationMenuItemSerializer(obj.items.all(), many=True).data
+
+
+class SiteSettingsSerializer(serializers.Serializer):
+    company_name = serializers.CharField()
+    contact_phone = serializers.CharField()
+    contact_email = serializers.CharField()
+    address = serializers.CharField()
+    icp_number = serializers.CharField()
+    icp_url = serializers.CharField()
+    wechat_account = serializers.CharField()
+    weibo_url = serializers.CharField()
+    wechat_qrcode = serializers.SerializerMethodField()
+
+    def get_wechat_qrcode(self, obj):
+        return serialize_image(obj.wechat_qrcode)