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/modules/billing/mod.rs

@@ -0,0 +1,10 @@
+mod handlers;
+pub mod service;
+
+use axum::Router;
+
+use crate::state::AppState;
+
+pub fn router() -> Router<AppState> {
+    handlers::router()
+}