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/forms/urls.py

@@ -0,0 +1,7 @@
+from django.urls import path
+
+from .views import LeadSubmitView
+
+urlpatterns = [
+    path("leads/", LeadSubmitView.as_view(), name="lead-submit"),
+]