price_per_unit was a Monetary field, so a realistic sub-cent rate like
$0.0075/core-hour was rounded to $0.01 on write, corrupting the rate.
Make it Float(16,6). Also stop _compute_billable from rounding the
overage amount to 2 decimals mid-calc — that lost the half-cent on
sub-cent rates and would drift against the source app, which keeps
usage amounts at 4 decimals and only rounds at the invoice total.
Now rounds to 6 dp (float-noise only); cent-rounding defers to the
invoice line. Exposed while building the NexaCloud importer.
3e0b531110