v0.9.7: Fix credit card posting signs (#31)

* Fix credit card posting signs for correct double-entry accounting

Credit card charges were posted as positive amounts on the liability
account and negative on expense accounts. This is inverted — charges
should be negative on the liability (increasing debt) and positive on
expenses. Payments were similarly inverted.

This also changes the split postings contract: categorizers should now
return positive amounts for expense splits to balance the negative
card posting.

Bump version to 0.9.7.

* Fix ruff lint: shorten long lines in tests