refactor(api): split Axum handlers into handler/service/repo modules

Keep the REST contract; move domain logic out of route files so checkout, fulfillment, and identity can be reused across customer, shop, and admin surfaces.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/api/src/money.rs

@@ -22,3 +22,49 @@ pub fn convert_minor(amount_minor: i64, from: &Currency, to: &Currency) -> Resul
         .to_i64()
         .ok_or_else(|| ApiError::BadRequest("amount out of range".into()))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use rust_decimal_macros::dec;
+
+    fn usd() -> Currency {
+        Currency {
+            code: "USD".into(),
+            name: serde_json::json!({"en": "US Dollar"}),
+            symbol: "$".into(),
+            exponent: 2,
+            is_base: true,
+            rate_to_base: dec!(1),
+            enabled: true,
+        }
+    }
+
+    fn jpy() -> Currency {
+        Currency {
+            code: "JPY".into(),
+            name: serde_json::json!({"en": "Yen"}),
+            symbol: "¥".into(),
+            exponent: 0,
+            is_base: false,
+            rate_to_base: dec!(150),
+            enabled: true,
+        }
+    }
+
+    #[test]
+    fn same_currency_is_identity() {
+        assert_eq!(convert_minor(199, &usd(), &usd()).unwrap(), 199);
+    }
+
+    #[test]
+    fn rejects_negative() {
+        assert!(convert_minor(-1, &usd(), &usd()).is_err());
+    }
+
+    #[test]
+    fn converts_via_base_half_up() {
+        // 1.00 USD -> 150 JPY at rate 150
+        assert_eq!(convert_minor(100, &usd(), &jpy()).unwrap(), 150);
+    }
+}