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/serializers.py

@@ -0,0 +1,7 @@
+from rest_framework import serializers
+
+
+class LeadSubmitSerializer(serializers.Serializer):
+    form_id = serializers.IntegerField()
+    data = serializers.DictField()
+    source_url = serializers.URLField(required=False, allow_blank=True, default="")