Add V006__incremental_ingest.sql to persist last_chunker_version and
last_embedding_version on the documents table. Wire both columns into
upsert_document (INSERT + ON CONFLICT UPDATE) and get_document (SELECT +
row mapper), replacing the previous hardcoded None. Add two round-trip
tests in tests/incremental_ingest.rs covering the set and None cases.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
