Token pricing runs to six decimal places. A thousand calls at $0.000015 is a number a float will quietly get wrong, and it will get it wrong in a direction nobody notices until a reconciliation fails.
We store micro-dollars as `bigint`, and every column ends `_micros` so a reader cannot mistake the unit. It is not clever. It is the kind of decision that is free on day one and touches every table on day four hundred.
The other half is the invariant: `available = balance − hold`, computed atomically, never assembled in application code. A subtraction done in a template is a second place for it to be wrong.
