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/state.rs

@@ -9,11 +9,12 @@ pub struct AppState {
 
 pub async fn build_state(config: &crate::config::Config) -> anyhow::Result<AppState> {
     let db = PgPool::connect(&config.database_url).await?;
+    assemble(config.clone(), db).await
+}
+
+/// Build state from an already-open pool (migrate + serve share one pool).
+pub async fn assemble(config: crate::config::Config, db: PgPool) -> anyhow::Result<AppState> {
     let client = redis::Client::open(config.redis_url.clone())?;
     let redis = redis::aio::ConnectionManager::new(client).await?;
-    Ok(AppState {
-        db,
-        redis,
-        config: config.clone(),
-    })
+    Ok(AppState { db, redis, config })
 }