Category CRUD endpoints

**Epic:** Budget

**Description**
Allow households to manage categories.

**Endpoints**
- `GET /api/v1/categories`
- `POST /api/v1/categories`
- `PATCH /api/v1/categories/:id`
- `DELETE /api/v1/categories/:id`

**Acceptance criteria**
- All operations scoped to household
- Cannot access categories from another household
- Request tests included