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/urls.py

@@ -0,0 +1,17 @@
+from django.urls import path
+
+from .api import (
+    NavigationMenuView,
+    PartnerListView,
+    SiteSettingsView,
+    TeamMemberListView,
+    TestimonialListView,
+)
+
+urlpatterns = [
+    path("team/", TeamMemberListView.as_view(), name="team-list"),
+    path("testimonials/", TestimonialListView.as_view(), name="testimonial-list"),
+    path("partners/", PartnerListView.as_view(), name="partner-list"),
+    path("navigation/", NavigationMenuView.as_view(), name="navigation-menu"),
+    path("site-settings/", SiteSettingsView.as_view(), name="site-settings"),
+]