refactor(api): split catalog module into product, category, brand

Rearranges the Rust backend by domain: `modules/catalog` bundled four
distinct concepts (Product, SKU, Category, Brand) behind one router
and one 392-line service file, unlike every other module in the
codebase which owns exactly one bounded aggregate.

Splits into `modules/product` (Product/SKU, publish lifecycle,
shop-scoped CRUD), `modules/category` (category tree, subtree query),
and `modules/brand` (brand list, admin replace-all). `Category`/`Brand`
move out of the shared `models.rs` into their owning modules;
`Product`/`Sku` stay since `favorite`/`flash_sale`/`group_buying`
reference them across modules. Purely internal restructuring — no
route, schema, or behavior changes.

Implements openspec change split-catalog-into-product-category-brand.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/api/src/modules/mod.rs

@@ -1,8 +1,9 @@
 pub mod account;
 pub mod address;
 pub mod billing;
+pub mod brand;
 pub mod cart;
-pub mod catalog;
+pub mod category;
 pub mod content;
 pub mod coupon;
 pub mod currency;
@@ -14,6 +15,7 @@ pub mod health;
 pub mod identity;
 pub mod order;
 pub mod points;
+pub mod product;
 pub mod shop;
 
 use axum::Router;
@@ -27,7 +29,9 @@ pub fn api_router() -> Router<AppState> {
         .merge(address::router())
         .merge(identity::router())
         .merge(currency::router())
-        .merge(catalog::router())
+        .merge(product::router())
+        .merge(category::router())
+        .merge(brand::router())
         .merge(content::router())
         .merge(cart::router())
         .merge(coupon::router())