docs: update ARCHITECTURE, GLOSSARY, db diagram for JourneyItem and Lesereise

- ARCHITECTURE.md: expand geschichte domain description — two subtypes
  (STORY/JOURNEY), JourneyItem ownership, ON DELETE SET NULL FK note
- GLOSSARY.md: add JourneyItem and Lesereise terms; update Geschichte
  entry to mention type discriminator
- db-relationships.puml: replace geschichten_documents with journey_items
  (ON DELETE CASCADE to geschichten, ON DELETE SET NULL to documents)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/architecture/db/db-relationships.puml

@@ -66,7 +66,7 @@ package "Supporting" {
   entity audit_log
   entity geschichten
   entity geschichten_persons
-  entity geschichten_documents
+  entity journey_items
 }
 
 ' Auth relationships
@@ -129,7 +129,7 @@ audit_log }o--o| documents : document_id
 geschichten }o--o| app_users : author_id
 geschichten_persons }o--|| geschichten : geschichte_id
 geschichten_persons }o--|| persons : person_id
-geschichten_documents }o--|| geschichten : geschichte_id
-geschichten_documents }o--|| documents : document_id
+journey_items }o--|| geschichten : geschichte_id (ON DELETE CASCADE)
+journey_items }o--o| documents : document_id (ON DELETE SET NULL)
 
 @enduml