Challenge 1: Write an App's urls.py — Possible Solution
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# catalog/urls.py

from django.urls import path
from . import views

urlpatterns = [
    path("", views.book_list, name="book-list"),
    path("<int:book_id>/", views.book_detail, name="book-detail"),
    path("category/<str:slug>/", views.by_category, name="book-category"),
]

WHY THIS WORKS
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- path("", views.book_list, name="book-list") matches the app's own root
  (whatever prefix it ends up included under) with an empty string — this
  is the standard way to express "the index/list page for this app."
- path("<int:book_id>/", ...) uses the built-in `int` converter, so
  `book_id` arrives at the view already converted to a Python int — a
  request for "/abc/" (non-numeric) won't even match this pattern, Django
  will just try the next one (or return a 404 if nothing matches).
- path("category/<str:slug>/", ...) captures everything after "category/"
  up to the next slash as a string named `slug` — `str` is also the
  default converter, so `<slug>` alone would behave identically, but
  being explicit keeps the pattern self-documenting.
- Every route has a distinct `name` — this is what makes Challenge 3's
  `reverse()` pattern possible later, and it costs nothing to add now.