Add CLI log file output

[mbsantiago]

Summary

Add support for saving CLI logs to a file with a new global --log-file option.

What changed

• Added --log-file to the root batdetect2 CLI command
• Updated logging setup to write to both:
  • the terminal
  • the specified log file
• Added CLI tests covering:
  • help output
  • log file creation and contents
• Updated docs with:
  • a note in the CLI reference
  • a tutorial example for using --log-file

Example

batdetect2 --log-file outputs/logs/cli.log process directory \
  path/to/audio \
  path/to/outputs