docs(adr): ADR-036 — geschichte responses are views, never entities

Captures the read-model invariant that was only living in a code comment:
views are assembled in-transaction, entities never cross the controller
boundary in the geschichte domain. CLAUDE.md §DTOs now names the exception
so the convention text stops contradicting the code.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/geschichte/GeschichteService.java

@@ -148,6 +148,9 @@ public class GeschichteService {
             g.setPublishedAt(LocalDateTime.now());
         }
         Geschichte saved = geschichteRepository.save(g);
+        // A freshly created Geschichte has no items by construction — items are only
+        // addable via the separate /items endpoints. Revisit if a create DTO ever
+        // accepts initial items.
         return toView(saved, List.of());
     }