`haiku-rag mcp` passes covers_set=True and _covering no longer refuses a
scope over several databases. search_documents, search_documents_by_image,
ask_question and analyze take `sources`; get_document takes `source`;
DocumentInfo carries `source`. format_citations gains include_source, which
ask_question sets from covers_multiple so citations name their database
only when the server covers several.
Refs #599
`resolve_document` and `find_document` selected a document through a listing,
then dropped its source and looked the id up across the set. Ids repeat between
copies of a database, so a title that matched in one could be answered by
another's document.
`get_document_by_id` and `get_chunk_by_id` join `get_picture_bytes` in taking an
optional `source`, and all three route it through `clients_covering`, so a name
the client does not cover raises `UnknownDatabaseError` rather than being
answered by the database it does cover. Without a source the reads are as they
were, answering from the first database in configured order that holds the id.
