diff --git a/docs/architecture/c4/l3-backend-3g-supporting.puml b/docs/architecture/c4/l3-backend-3g-supporting.puml
index ea08f206..11f45b1f 100644
--- a/docs/architecture/c4/l3-backend-3g-supporting.puml
+++ b/docs/architecture/c4/l3-backend-3g-supporting.puml
@@ -18,6 +18,7 @@ System_Boundary(backend, "API Backend (Spring Boot)") {
     Component(sseRegistry, "SseEmitterRegistry", "Spring Component", "In-memory ConcurrentHashMap of Spring SseEmitter instances per user. Handles registration, deregistration, and JSON event broadcasts.")
     Component(geschCtrl, "GeschichteController", "Spring MVC — /api/geschichten", "CRUD for publishable stories (STORY) and reading journeys (JOURNEY). Returns GeschichteSummary projections for list; full Geschichte with JourneyItems for detail. Requires BLOG_WRITE permission for write operations.")
     Component(geschSvc, "GeschichteService", "Spring Service", "Manages story lifecycle (DRAFT → PUBLISHED with timestamp). Supports two subtypes: STORY (prose) and JOURNEY (ordered JourneyItem sequence). Sanitizes HTML body with an allowlist policy.")
+    Component(journeyItemSvc, "JourneyItemService", "Spring Service", "Manages journey item lifecycle: append (100-item cap), updateNote (three-way PATCH), delete, and reorder (DEFERRABLE position swap). Enforces JOURNEY-type guard on append.")
     Component(exHandler, "GlobalExceptionHandler", "Spring @RestControllerAdvice", "Converts DomainException, validation errors, and generic exceptions to ErrorResponse JSON with machine-readable ErrorCode and HTTP status.")
 }
 
@@ -38,6 +39,9 @@ Rel(notifCtrl, notifSvc, "Delegates to")
 Rel(notifCtrl, sseRegistry, "Registers client SSE connection")
 Rel(notifSvc, sseRegistry, "Broadcasts events to connected clients")
 Rel(geschCtrl, geschSvc, "Delegates to")
+Rel(geschCtrl, journeyItemSvc, "Delegates journey item CRUD")
+Rel(geschSvc, journeyItemSvc, "Delegates getItems()")
+Rel(journeyItemSvc, db, "Reads / writes journey_items", "JDBC")
 Rel(auditSvc, db, "Writes audit_log", "JDBC")
 Rel(auditQuery, db, "Reads audit_log", "JDBC")
 Rel(notifSvc, db, "Reads / writes notifications", "JDBC")