Introduces SyncEngine that wraps IngestionPipeline with a SQLite state
database (sync_state.db) for checkpoint/resume: cursors and item counts
are persisted after every 100-document batch and on completion/error.
Adds 4 tests covering single-connector ingestion, checkpoint accuracy,
unsynced-connector None return, and multi-connector source filtering.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
