Money is a bigint, and the column name says so

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.

// $3.00 per million tokens, as an integer of micro-dollars
const INPUT_PER_MTOK = 3_000_000n

// what 1,204 input tokens cost, with no floating point anywhere on the path
const costMicros = (INPUT_PER_MTOK * 1_204n) / 1_000_000n  // → 3_612n  ($0.003612)

// the float version, for comparison
const asFloat = (3.0 / 1_000_000) * 1204            // 0.0036119999999999997
// …and a thousand of those is off by enough to fail a reconciliation

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.

  wallet
  ┌──────────────────────────────────────────────┐
  │  balance_micros        34_210_000   $34.21   │  what they put in
  │  hold_micros            6_000_000   $ 6.00   │  reserved by calls in flight
  │  ──────────────────────────────────────────  │
  │  available            = balance − hold        │  computed, atomically
  │                        28_210_000   $28.21   │  never a column
  └──────────────────────────────────────────────┘

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.