Add insertPrices() to scraperService that performs bulk upsert of
daily unit price records to the prices collection. Key behaviors:
- Uses bulkWrite with updateOne/upsert ops for atomic batch writes
- Compound key (unit_code + date_checked) ensures idempotent re-runs
- Filters out units with null prices before writing
- Stores price, last_updated, and data_source fields per record
- Uses ordered:false for parallel write performance
- Logs warnings for empty/all-null input without hitting the database
- Propagates bulkWrite errors with detailed logging
Includes 30 tests covering filtering, upsert behavior, composite key
logic, field validation, idempotency, error handling, and logging.
