Add --file option for single file processing

[uwe-schwarz]

Add --file option for single file processing

This PR adds a new command-line option --file that allows processing a single PDF file instead of scanning an entire directory.

Changes:

• Added a new --file option to the argument parser
• Made directory and file options mutually exclusive
• Added logic to process a single file when the --file option is specified
• Preserved all existing functionality for directory processing

Usage:

To process a single file:
uv run scan.py --file "/path/to/file.pdf"

To process a directory (unchanged):
uv run scan.py "/path/to/directory"
or
uv run scan.py --directory "/path/to/directory"