Update Imported Document

Partial update for a document, identified by source_id and remote_id.

Source can be supplied as url OR content (base64), not both. A PATCH with neither is a metadata-only update (display_name, owners, …). A source change (different URL, or content supplied) creates a new document version and reprocesses the document.

author is intentionally not updatable: it's set at ingest time and may be linked to a Sybill person. Callers that need to change the author should delete and re-ingest.