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/brand/mod.rs

@@ -0,0 +1,11 @@
+mod dto;
+mod handlers;
+pub mod service;
+
+use axum::Router;
+
+use crate::state::AppState;
+
+pub fn router() -> Router<AppState> {
+    handlers::router()
+}