Add Category model and relate it to Tasks

[Navashub]

Description

Tasks should be organizable by category (e.g. "Work", "Personal", "Learning"). Build the Category model, its CRUD, and link it to Task.

What to do

• Create a Category SQLAlchemy model in models.py: id, name (unique, not null)
• Add a category_id foreign key to the Task model (nullable — tasks don't have to have a category)
• Add a relationship from Task to Category
• Create Pydantic schemas: CategoryCreate, CategoryResponse
• Create routers/categories.py with full CRUD: POST, GET /, GET /{id}, DELETE /{id}
• Register the router in main.py
• Include category (nested) in TaskResponse

Acceptance Criteria

• POST /categories/ creates a category
• GET /categories/ lists all categories
• POST /tasks/ accepts an optional category_id
• GET /tasks/ includes the category object nested inside each task response

Difficulty

🟡 Intermediate