[maintainer] Ingest nominator counts per validator + expose in the directory

[JSONbored]

The nominator-count column (and 24h change) is a taostats staple but metagraphed has no nominator/delegation data: neurons carries only stake/emission/trust, and delegation is not indexed in the snapshot tier. Requires a new ingestion source + schema, so it is maintainer-owned, not a bounty.

Deliverables

• Decide the source (upstream delegation API vs. chain delegation pallet) and add a nominator_count column via a contiguous migration
• Extend the refresh cron/staging path to populate nominator_count (and, if in scope, a prior-snapshot delta for 24h change)
• Expose nominator_count in buildGlobalValidatorEntry + /api/v1/validators + contracts + OpenAPI
• Add the nominator column to validators.index.tsx once the field ships
• Tests for ingestion + serving; null-safe when the source lacks a hotkey

References

• repo/src/metagraph-neurons.mjs:157 (buildGlobalValidatorEntry - no nominators)
• repo/migrations/ (add contiguous NNNN_*.sql)
• repo/workers/request-handlers/staging.mjs (ingestion path)

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size: M · maintainer-only (owner)

[linear-code]

JSO-2087