Use gnomAD Toolbox for variant frequencies

[Copilot]

Replace unreliable client-side gnomAD GraphQL requests with backend gnomAD Toolbox retrieval while retaining locally stored frequency data as an outage fallback.

• Backend retrieval

  • Add gnomad-toolbox dependency.
  • Query the gnomAD v4.1 joint dataset per variant.
  • Map all existing frequency fields: af_tot, ac_tot, an_tot, hom_tot, and hemi_tot.
  • Preserve the existing gnomadFrequencies response shape, including id and nested variant.

• Fallback behavior

  • Fall back to GenomicGnomadFrequency when Toolbox/Hail access fails.
  • Add an API error note when local data is returned.

• Frontend

  • Consume gnomadFrequencies from the variant endpoint.
  • Remove the direct GraphQL request and artificial loading delay.

gnomadFrequencies = {
    "id": variant.id,
    "variant": VariantSerializer(variant).data,
    **get_gnomad_toolbox_frequencies(variant.variant_id),
}

• Fixes move from graphql API requests to gnomAD toolbox #64