Batch progress callback / event hook

[ArkNill]

Feature Request

BatchProcessor.process_directory() currently runs silently. Add a progress callback:

def on_progress(current: int, total: int, path: str, result: ExtractionResult):
    print(f"[{current}/{total}] {path}: {'OK' if result.data else 'FAILED'}")

processor.process_directory("./docs/", schema=InvoiceSchema, on_progress=on_progress)

Motivation

Useful for long-running batch jobs and integration with progress bars.