Screen a batch of names against the sanctions corpus

Screen up to 1000 names in one request, each against the OFAC SDN corpus, returning a per-item screening signal (clear, review, or hit) plus an aggregate summary. Each item is screened with the same fail-CLOSED staleness and partial-coverage downgrade as POST /screen. Billing: each item counts as one billable screen (N items = N screens). Fail-CLOSED: if no sanctions list is loaded the whole batch returns 503 (never a per-item false clear).