Challenge 1: Define and Migrate a Model — Possible Solution
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# catalog/models.py

from django.db import models

class Author(models.Model):
    name = models.CharField(max_length=150)
    bio = models.TextField(blank=True, null=True)
    birth_year = models.IntegerField(blank=True, null=True)

    def __str__(self):
        return self.name

# Commands to create and apply the migration:

python manage.py makemigrations catalog
# Migrations for 'catalog':
#   catalog/migrations/0002_author.py
#     - Create model Author

python manage.py migrate
# Applying catalog.0002_author... OK

WHY THIS WORKS
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- name uses CharField with a max_length, appropriate for a short, bounded
  piece of text — a person's name is never going to need TextField's
  unlimited length.
- bio uses TextField (correct for potentially long, free-form text) with
  both blank=True and null=True, since a bio is genuinely optional: an
  author record should be valid with no bio at all, both at the database
  level (null) and at the form/validation level (blank).
- birth_year uses IntegerField, also optional with blank=True and
  null=True — some authors' birth years may simply be unknown, so it
  shouldn't be a required field.
- __str__ returns the author's name so that Author objects display
  meaningfully in the admin site and Django shell, instead of the default
  "Author object (1)".
- makemigrations is run with the app name (catalog) explicitly, though
  running it with no arguments would also detect this new model — being
  explicit is a good habit in a project with multiple apps, so you know
  exactly which app's changes are being captured.