fix(core): return 404 from marker_preview when ?id= is missing, non-int, or unknown

[SAY-5]

What

marker_preview in src/core/views/views.py calls:

@require_http_methods(["GET"])
def marker_preview(request):
    marker_id = request.GET.get("id")
    marker = Marker.objects.get(id=marker_id)
    ...

marker_id is whatever came off the query string: None, a numeric string, or arbitrary junk. Marker.objects.get(id=marker_id) turns every non-happy-path into an HTTP 500 via Django's default handler:

1. ?id missing → marker_id is None → ORM issues id IS NULL → DoesNotExist.
2. ?id=abc (or any non-numeric junk bots send) → DB driver rejects the integer cast → ValueError / DataError.
3. ?id=99999 (deleted / never existed) → DoesNotExist.

All three should be 404s, not server errors. The /marker/preview endpoint is unauthenticated and reachable from marker thumbnails in the collection UI, so stale links and crawlers reliably log 500s in the error tracker, drowning real bugs.

Fix

Same pattern already used for exhibit_detail and related_content in this module: coerce the id to int first (Http404 on failure), then fetch via get_object_or_404.

marker_id = request.GET.get("id")
try:
    marker_id = int(marker_id)
except (TypeError, ValueError):
    raise Http404
marker = get_object_or_404(Marker, id=marker_id)

get_object_or_404 and Http404 are already imported from django.shortcuts / django.http at the top of the file; no new imports needed. Behaviour for a valid, existing marker id is unchanged.

Fixes #847