feat(items): support drag-and-drop reordering and contextual insert

Implement drag-and-drop sorting in the items grid
Add right-click context menu to insert new items before or after
Update backend to process insertion anchors during item creation

frontend/src/services/api.ts

@@ -800,6 +800,8 @@ export interface ItemPayload {
   acquisitionMethodIds: number[];
   tagIds: number[];
   imagePath: string;
+  insertBeforeItemId?: number | null;
+  insertAfterItemId?: number | null;
 }
 
 export interface AncientArtifactPayload {