Get fan insights in bulk (POST batch)

Returns detailed insights for up to 100 fans in a single request, keyed by the input fan UUID.

This is the high-cap, JSON-body counterpart to GET /insights/fans (capped at 20 via query string). It is intended for initial-sync flows that need to hydrate many fans without per-fan round trips.

Per-key errors are reported inside the 200 response body so a single forbidden or missing fan never collapses the whole request — consumers can keep partial results and only retry the failing keys.

<Note>Maximum 100 fan UUIDs per request. Failed keys are reported as { "error": "forbidden" | "not_found" | "internal" }.</Note>