feat: backend MVP (auth/rbac, catalog, orders, fulfillment, invoices) + specs + scaffolds

apps/api/src/money.rs

@@ -0,0 +1,23 @@
+use rust_decimal::prelude::*;
+use rust_decimal::Decimal;
+
+use crate::error::ApiError;
+use crate::models::Currency;
+
+/// Convert integer minor units between currencies via base rates.
+/// rate_to_base = units of currency per 1 unit of base. Rounds half-up
+/// to whole minor units of the target currency.
+pub fn convert_minor(amount_minor: i64, from: &Currency, to: &Currency) -> Result<i64, ApiError> {
+    if amount_minor < 0 {
+        return Err(ApiError::BadRequest("amount_minor must be >= 0".into()));
+    }
+    let amount = Decimal::from(amount_minor);
+    let from_scale = Decimal::from(10i64.pow(from.exponent as u32));
+    let to_scale = Decimal::from(10i64.pow(to.exponent as u32));
+    // minor(from) -> major(from) -> major(base) -> minor(to)
+    let base_major = amount / from_scale / from.rate_to_base;
+    let target = (base_major * to.rate_to_base * to_scale).round_dp(0);
+    target
+        .to_i64()
+        .ok_or_else(|| ApiError::BadRequest("amount out of range".into()))
+}