Return structural metadata for a document: filing type, source document type, total character count, named sections with character counts, and a recommended_call hint that guides whether to fetch the whole document or a specific section next. The endpoint never returns the document body. Documents whose filing type is outside the allowlist return 200 with recommended_call set to not_supported and reason populated. Earnings call transcripts are excluded from whole-document access and return 403; use /documents/keyword-search to query transcript content.