Return 404 instead of 500 on missing marker_preview (#847)

[vjpixel]

marker_preview fetched Marker.objects.get(id=marker_id) directly, so an invalid or missing id raised DoesNotExist/ValueError through to a 500. Switch to the already-imported get_object_or_404. Pablo: LGTM.

Closes #847

https://claude.ai/code/session_01XC1THLWgnGXGf5wgRhdyvB

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[vjpixel]

Self-review

Verdict: ✅

Marker.objects.get(id=marker_id) → get_object_or_404(Marker, id=marker_id). Django 6's get_object_or_404 natively catches ValueError/TypeError/DoesNotExist, so missing/garbage ?id= values cleanly return 404 instead of 500.

vs duplicate #915: Functionally equivalent on Django 6. #915 adds an explicit int() coercion before the call — slightly more defensive (works on Django ≤ 4.0 and signals intent more clearly). Either approach works given the pinned Django 6 dependency.

Duplicate: #915.

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[vjpixel]

Closing in favor of @SAY-5's #915 (same fix with explicit int coercion, slightly more defensive, opened first). See review there.

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