feat: add products/cases page models and forms lead-capture app

- apps/products: ProductIndexPage/ProductPage
- apps/cases: CaseStudyIndexPage/CaseStudyPage
- apps/forms: FormDefinition/FormDefinitionField/Lead + submit API
- core/blocks: CaseStudyBlock, FormBlock (with API representation)
- register new apps, custom leads API route, migrations

apps/cases/apps.py

@@ -0,0 +1,8 @@
+from django.apps import AppConfig
+
+
+class CasesConfig(AppConfig):
+    default_auto_field = "django.db.models.BigAutoField"
+    name = "apps.cases"
+    label = "cases"
+    verbose_name = "客户案例"