feat: implement single POST upsert API with automatic ID generation

- Replace separate POST/PUT endpoints with unified POST upsert
- Add automatic content ID generation from element context when no ID provided
- Implement version history preservation before content updates
- Add element context support for backend ID generation
- Update frontend to use single endpoint for all content operations
- Enhanced demo site with latest database content including proper content IDs

cmd/serve.go

@@ -130,7 +130,7 @@ func runServe(cmd *cobra.Command, args []string) {
 	contentRouter := apiRouter.PathPrefix("/content").Subrouter()
 	contentRouter.HandleFunc("/bulk", contentHandler.GetBulkContent).Methods("GET")
 	contentRouter.HandleFunc("/{id}", contentHandler.GetContent).Methods("GET")
-	contentRouter.HandleFunc("/{id}", contentHandler.UpdateContent).Methods("PUT")
+
 	contentRouter.HandleFunc("", contentHandler.GetAllContent).Methods("GET")
 	contentRouter.HandleFunc("", contentHandler.CreateContent).Methods("POST")