fix(review): address PR #788 review blockers

- GlobalExceptionHandler maps uq_journey_items_geschichte_position constraint
  violation to HTTP 409 JOURNEY_ITEM_POSITION_CONFLICT
- JourneyItemService.reorder() rejects duplicate IDs before set-equality check
  to prevent silent position overwrite
- JourneyItemRepository removes orphaned findAllByGeschichteId method
- GeschichteView removes stale com.fasterxml.jackson import
- Tests: add appendItem_returns409_on_position_conflict (controller),
  reorder_returns400_when_itemIds_contain_duplicates (service)
- Fix JourneyItemIntegrationTest compilation after repository cleanup
- db-orm.puml annotates journey_items position with CHECK + UNIQUE DEFERRABLE

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

docs/architecture/db/db-orm.puml

@@ -376,8 +376,10 @@ package "Supporting" {
     --
     geschichte_id : UUID <<FK>>
     document_id : UUID <<FK>>
-    position : INTEGER NOT NULL
+    position : INTEGER NOT NULL CHECK (position > 0)
     note : TEXT
+    ==
+    UNIQUE (geschichte_id, position) DEFERRABLE INITIALLY DEFERRED
   }
 }