feat: --get needs a --force command for usecase updates #38

Problem Statement

If you are using a venv (not an issue with uvx), when --get is first used, logstory copies the usecase/$USECASE from:

LOGSTORY_USECASES_BUCKETS=file://...

to:

venv/lib/python3.12/site-packages/logstory/usecases/$USECASE

Once copied, the next time it runs, it finds that $USECASE dir already exists and does not copy again. This is confusing when you are developing a usecase and the changes you make are not detected.

Proposed Solutions

1. Add a --force flag that copies regardless of the presence of the $USECASE dir in site-packages.
2. Create a checksum for the contents of the EVENTS and ENTITIES dirs and keep it in the metadata. Regenerate the checksums whenever --get is used and if different, copy the files.
3. ...