fix(blog): return 404 from post_detail when pk does not match a post

[SAY-5]

What

post_detail in src/blog/views.py called:

def post_detail(request, pk):
    post = Post.objects.prefetch_related("images", "categories").get(pk=pk)
    ...

QuerySet.get(pk=pk) raises Post.DoesNotExist when the pk does not match a row. The view did not catch it, so Django's default handler turned every missing-post request into an HTTP 500 Internal Server Error.

The route blog/post/<int:pk>/ is public, so any stale permalink, crawler, or user who types a pk that has since been deleted triggers a 500. On top of being wrong from an HTTP-semantics angle (it should be 404), it also pollutes Sentry / the server's error log with noise that masks real 500s.

Fix

Swap .get(pk=pk) for django.shortcuts.get_object_or_404 against the same prefetched queryset:

from django.shortcuts import get_object_or_404, render

def post_detail(request, pk):
    post = get_object_or_404(
        Post.objects.prefetch_related("images", "categories"),
        pk=pk,
    )
    ...

get_object_or_404 delegates to the same .get() call under the hood, so behaviour for any pk that does resolve is identical (same prefetching, same queryset). The only observable change is that a missing pk now returns a clean 404 Not Found.

Fixes #856