Add database engine, session, and ORM models

[ddvlamin]

Description

Create the async SQLAlchemy setup and ORM models for shopping lists and items.

Files

• Create: app/__init__.py
• Create: app/database.py — async engine + session factory + get_db dependency
• Create: app/models.py — ShoppingList and ShoppingListItem ORM models

Details

• Use async SQLAlchemy with aiosqlite
• ShoppingList: id, name, created_at, updated_at
• ShoppingListItem: id, shopping_list_id (FK), name, quantity, unit, is_checked, created_at
• Relationship with cascade delete on items

Acceptance Criteria

• Models import without errors
• Tables can be created from Base.metadata