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/models.rs

@@ -86,24 +86,6 @@ pub enum ProductStatus {
     Unpublished,
 }
 
-#[derive(Debug, Serialize, sqlx::FromRow)]
-pub struct Category {
-    pub id: Uuid,
-    pub parent_id: Option<Uuid>,
-    pub name: serde_json::Value,
-    pub slug: String,
-    pub position: i32,
-}
-
-#[derive(Debug, Serialize, sqlx::FromRow)]
-pub struct Brand {
-    pub id: Uuid,
-    pub name: serde_json::Value,
-    pub slug: String,
-    pub position: i32,
-    pub active: bool,
-}
-
 #[derive(Debug, Clone, Serialize, sqlx::FromRow)]
 pub struct Product {
     pub id: Uuid,