Implement comprehensive Categories system for todo management

- Add Category model with id, name, description, color fields
- Update Todo model with category_id foreign key relationship
- Create Category CRUD operations with full validation
- Add Category API endpoints with complete REST functionality
- Update Todo API and CRUD to support category filtering
- Create Alembic migration to add categories table
- Clean up code structure and remove unused references
- Update all import files and maintain proper relationships

Categories system enables better todo organization and filtering capabilities.

app/schemas/todo.py

@@ -35,6 +35,7 @@ class Todo(TodoBase):
     created_at: datetime
     updated_at: Optional[datetime] = None
     category: Optional["Category"] = None
+    project: Optional["Project"] = None
 
     class Config:
         from_attributes = True