Bulk-write documents on Notion import

The Notion importer (#275) creates documents one at a time. Each is a separate insert, plus its meta, plus a term assignment, so a large database means a lot of round-trips.

**Proposed solution:** batch the writes. Insert documents in chunks, prime caches once, and write meta and trait terms in bulk instead of per document.

Start with the Notion importer described in issue #275 and trace its per-document insert, metadata write, and term assignment. Identify the existing cache initialization and database write paths; done means documents are inserted in chunks, caches are primed once, and metadata and trait terms are written in bulk without changing import results.

Written by the indexing model from the issue text.