remove notebooks

- Delete notebooks/ (personal scratch artifacts; gitignored)
- Move nbformat to --extra migrate; lazy imports in migrate.py and
  cli.py so base CLI loads without it
- Restore langchain-text-splitters in core (chunking dependency)

src/ocr_pipeline/utils/migrate.py

@@ -6,8 +6,6 @@ import platform
 from pathlib import Path
 from typing import Any
 
-import nbformat
-
 from ocr_pipeline.utils.logging import get_logger
 
 logger = get_logger(__name__)
@@ -36,6 +34,8 @@ def _get_pictures_dir() -> Path:
 def parse_notebook_for_images(notebook_path: Path) -> list[Path]:
     image_paths = []
     try:
+        import nbformat
+
         with open(notebook_path) as f:
             nb = nbformat.read(f, as_version=4)