sequenceDiagram
  autonumber
  actor Client
  participant App as Flask App
  participant API as Smorest Api
  participant BP as Category Blueprint
  participant JWT as JWT Ext
  participant DB as SQLAlchemy/DB

  rect rgb(240,248,255)
  note over Client,DB: List categories (GET /categories)
  Client->>App: GET /categories
  App->>API: dispatch request
  API->>BP: CategoryCollection.get
  BP->>DB: Category.query.all()
  DB-->>BP: categories
  BP-->>Client: 200 JSON
  end

  rect rgb(245,255,240)
  note over Client,DB: Create category (POST /categories) [protected]
  Client->>App: POST /categories (JSON)
  App->>API: dispatch
  API->>JWT: verify access token
  JWT-->>API: ok / error
  alt token valid
    API->>BP: CategoryCollection.post
    BP->>DB: validate subcategory IDs
    DB-->>BP: ok / missing
    alt valid
      BP->>DB: insert Category, commit
      DB-->>BP: created
      BP-->>Client: 201 JSON
    else invalid
      BP-->>Client: 422 error
    end
  else invalid token
    API-->>Client: 401 error
  end
  end

  rect rgb(255,250,240)
  note over Client,DB: Paginated products (GET /categories/{id}/products?page=n)
  Client->>App: GET /categories/123/products?page=2
  App->>API: dispatch
  API->>BP: CategoryProducts.get
  BP->>DB: exists(Category.id==123)?
  alt exists
    BP->>DB: query products, paginate (10/page)
    DB-->>BP: items + meta
    BP-->>Client: 200 JSON
  else not found
    BP-->>Client: 404
  end
  end