perf: renderUI repeatedly queries list_documents() during processing

[seanthimons]

Description

In R/mod_document_notebook.R (~line 589), output$document_list <- renderUI(...) calls list_documents(con(), nb_id) on every render cycle. During streaming/processing, this causes repeated DB queries.

Current behavior

Each time doc_refresh() triggers a re-render (which happens during chat streaming), list_documents() hits the database again.

Expected behavior

Cache the document list in a reactiveVal and only refresh it when documents are actually added/removed — not on every render cycle.

Fix suggestion

doc_list <- reactiveVal(data.frame())

observe({
  req(con(), current_nb_id())
  doc_list(list_documents(con(), current_nb_id()))
}) |> bindEvent(doc_refresh())

output$document_list <- renderUI({
  docs <- doc_list()
  # ... render using cached docs
})

Context

Found during PR #156 review. Not a correctness issue — performance optimization for larger notebooks.