Security-level holdings for one ETF from the filed N-PORT store.

Public, per the tier doctrine above: this reads PG only and makes no upstream call, so it sits with /api/etf-list rather than /api/etf-data.

Why this route exists: v230 landed etf_nport_holdings (284,858 rows / 147 series) plus a typed read helper, and the helper had NO callers -- the store's only production read was an EXISTS count. A dataset nobody can query answers no question.

Honest-empty on every miss: a non-ETF ticker, an ETF with no series_id in the catalog, and a series with no filed period yet all return holdings=[] rather than an error, because none of those is a failure.