Add Pydantic request/response schemas

[ddvlamin]

Description

Create Pydantic models for API request validation and response serialization.

Files

• Create: app/schemas.py

Schemas Needed

• ShoppingListCreate, ShoppingListResponse, ShoppingListDetailResponse (with items)
• ShoppingListItemCreate, BulkItemsCreate, ShoppingListItemResponse, ShoppingListItemUpdate

Details

• All response schemas use model_config = {"from_attributes": True} for ORM compatibility
• ShoppingListDetailResponse includes items list
• ShoppingListItemUpdate uses optional fields for partial updates

Acceptance Criteria

• Schemas import correctly
• Validation rejects empty name on create