Add upsertUnits() to scraperService that performs bulk upserts of
apartment unit data using MongoDB bulkWrite with unordered operations.
Key behaviors:
- Uses unit_code as the unique key for matching existing units
- Sets last_scraped timestamp and data_source on every upsert
- Uses $setOnInsert for first_seen to preserve original insert time
- Handles empty arrays gracefully with early return
- Uses ordered:false for parallel execution and partial failure tolerance
- Logs result counts (matched, modified, upserted) on success
- Propagates errors with detailed logging on failure
Includes 23 tests covering bulk operations, upsert behavior, field
handling, empty input, error scenarios, and return values.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
