RAG auto-indexing now applies backpressure to its background worker pool. The `ThreadPoolExecutor` previously had an unbounded internal queue; under sustained upload bursts (possible under the configurable upload rate cap from #3935), this could queue thousands of indexing jobs and exhaust memory. The pool is now capped at 100 in-flight + queued jobs; submissions over the cap are dropped with a clear log warning ("Auto-index queue saturated… trigger a manual reindex if needed"). Documents still upload successfully — only the *automatic* indexing is skipped, and users can manually reindex via the UI.
