Backend: POST /api/v1/tools/:toolId/batch accepts multiple files +
settings, processes via p-queue with CONCURRENT_JOBS concurrency limit,
streams ZIP response using archiver. Tool registry in tool-factory
enables batch to reuse any registered tool's process function. SSE
endpoint at GET /api/v1/jobs/:jobId/progress provides real-time updates.
Handles partial failures gracefully, preserves filenames, deduplicates
collisions. Frontend: use-batch-processor hook handles upload, SSE
progress tracking, and automatic ZIP download.
