Dedicated thread pool (core=1, max=2, queue=200) with CallerRunsPolicy for back-pressure. Keeps thumbnail rendering off the shared taskExecutor used by OCR and out of the AbortPolicy queue that drops work on overflow. Quick-upload batches (15+ files) now apply back-pressure instead of silently dropping thumbnail jobs. Refs #307 Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>