fix(core): validate and 404 before touching edit_marker / edit_object payloads

[SAY-5]

What

Both edit_marker and edit_object in src/core/views/views.py had the same bug-shaped logic:

def edit_marker(request):
    index = request.GET.get("id", "-1")
    model = Marker.objects.get(id=index)

    model_data = {
        "source": model.source,
        ...
    }

    if not model or model.owner != Profile.objects.get(user=request.user):
        raise Http404

Three things are wrong:

1. 500 on unknown/garbage id. Marker.objects.get(id=index) raises DoesNotExist for unknown ids and ValueError / DataError for non-numeric values. The default "-1" falls back to a DB query that returns nothing and again raises DoesNotExist. All three surface as HTTP 500.
2. model_data built before the permission check. The dict is filled from the row's fields (source, created, author, patt, title) before we know if the requester owns the row. Today the data never escapes because Http404 is raised two lines later, but the ordering is exactly the kind that breaks the moment someone adds logging, a metric, or serialisation above the guard.
3. if not model is dead code: .get() either returns an instance or raises, so the sentinel branch can never fire.

Fix

• Coerce index to int first, Http404 on failure.
• Fetch with get_object_or_404 so unknown id → 404.
• Do the owner != request.user.profile check before building model_data.
• Drop the dead if not model branch.

edit_object gets the same treatment (same bug in the same shape).

def edit_marker(request):
    index = request.GET.get("id", "-1")
    try:
        index = int(index)
    except (TypeError, ValueError):
        raise Http404
    model = get_object_or_404(Marker, id=index)
    if model.owner != request.user.profile:
        raise Http404
    model_data = { ... }

Matches the pattern already used by create_or_edit_exhibit and edit_artwork further down this file.

Fixes #846