feat(variant): show gnomAD population frequency on the variant page

[bencap]

Fills the annotations card's "Population Frequency" column, until now a "Data coming soon" placeholder. gnomAD is linked to a single mapped variant by ClinGen allele ID, so the frequency is a direct assertion about the measured allele rather than an aggregate over related variants.

• Add MvGnomadSummary: AF with the AC/AN behind it, FAF95 with the genetic ancestry group attaining it, and a deep link to the gnomAD variant page, bottom-aligned so provenance lines up across the card's columns.
• Read the record from the score set's variant data CSV via gnomadFromVariantRow, which normalises the namespace's NA sentinels. A variant absent from gnomAD reports as having no record, not zero frequency.
• Derive GnomadFrequency from the generated OpenAPI schema, so renaming or retyping a field on the API model breaks compilation here.
• Split HISTOGRAM_VARIANT_DATA_NAMESPACES into SCORE_SET_CHART_NAMESPACES and VARIANT_PAGE_NAMESPACES. One list fed two screens under a name describing neither, and only the variant page reads gnomAD, so the score set charts no longer fetch seven unused columns on every row.

Requires the widened gnomad CSV namespace in the API, see VariantEffect/mavedb-api#836.

[coveralls]

Coverage is 10.159% — feature/bencap/447/gnomad-allele-frequency into feature/bencap/variant-level-csv-download. No base build found for feature/bencap/variant-level-csv-download.