Wrap camera.read_frame() call in try/except to catch RuntimeError and
return a 503 Service Unavailable response with structured error JSON
(error + detail fields) instead of letting the exception propagate
as an unhandled 500. This gives kiosk operators a friendly, actionable
error message when the camera becomes unavailable.
Added FailingCamera test fixture and regression test that verifies
the 503 response with expected error details.
