feat: add solutions app, fix HomePage subpage_types, add pytest test suite

- apps/solutions: SolutionIndexPage/SolutionPage (mirrors products/cases)
- apps/home: HomePage.subpage_types now includes products/solutions/cases index pages (previously only blog, blocking admin page creation)
- pytest.ini + conftest.py (root_page/home_page fixtures)
- unit tests for core (SEOablePage, FormBlock API repr), products, cases, forms (lead submit API)

apps/cases/tests.py

@@ -0,0 +1,50 @@
+"""apps.cases 单元测试：页面树结构与列表排序。"""
+import pytest
+from django.utils import timezone
+
+from apps.cases.models import CaseStudyIndexPage, CaseStudyPage
+
+pytestmark = pytest.mark.django_db
+
+
+@pytest.fixture
+def case_index(home_page):
+    index = CaseStudyIndexPage(title="客户案例", slug="cases", intro="客户的成功案例")
+    home_page.add_child(instance=index)
+    return index
+
+
+def test_case_study_page_can_be_created_under_index(case_index):
+    case = CaseStudyPage(
+        title="某集团数字化转型",
+        slug="some-group",
+        client_name="某集团",
+        industry="制造业",
+        published_at=timezone.now(),
+    )
+    case_index.add_child(instance=case)
+
+    assert CaseStudyPage.objects.live().descendant_of(case_index).count() == 1
+
+
+def test_case_index_context_orders_by_published_at_desc(case_index):
+    older = CaseStudyPage(
+        title="案例一",
+        slug="case-1",
+        client_name="客户一",
+        published_at=timezone.now() - timezone.timedelta(days=10),
+    )
+    case_index.add_child(instance=older)
+
+    newer = CaseStudyPage(
+        title="案例二",
+        slug="case-2",
+        client_name="客户二",
+        published_at=timezone.now(),
+    )
+    case_index.add_child(instance=newer)
+
+    context = case_index.get_context(request=None)
+    titles = [c.title for c in context["case_studies"]]
+
+    assert titles == ["案例二", "案例一"]