Resolve a content hash (sha256 of the source bytes) to its document.

Hash resolution: proof bundles carry the sha256 content_hash, so a consumer holding a proof can resolve the hash back to the document record without guessing an id. Bounded and read-quota-safe: the lookup is an index read on idx_books_sha256 (created by the ingestion pipeline's schema ensure), fanned out across the serving shards like resolve, first hit wins. Access-aware result shape — same fields as /api/resolve per-document results, including the proof bundle, never raw full text.