Performance: Scope llm index updates to actually modified documents (#13322)

* Perf: scope bulk_update_documents' LLM index refresh to the edited document ids instead of the whole library

* Perf: select_related/prefetch_related for the scoped LLM index batch

* Perf: select_related/prefetch_related for the full LLM index rebuild path

* Fix: address Copilot review findings on LLM index scoping

src/documents/tasks.py

@@ -311,6 +311,7 @@ def sanity_check(*, raise_on_error: bool = True) -> str:
 def bulk_update_documents(document_ids) -> None:
     from documents.search import get_backend
 
+    document_ids = list(document_ids)
     documents = Document.objects.filter(id__in=document_ids)
 
     for doc in documents:
@@ -331,6 +332,7 @@ def bulk_update_documents(document_ids) -> None:
     if ai_config.llm_index_enabled:
         update_llm_index(
             rebuild=False,
+            document_ids=document_ids,
         )