With WithStore, the Find a Tender and mock workers save each run's
opportunities with SaveOpportunities and log what changed; a failed save
fails the run, so the checkpoint stays put and the next run retries.
Without a store nothing is persisted. The mock's opportunities are saved
with Source "FTSMock" rather than the converter's "FindATender".
fetcher service and job-search serve open the store at database.file
(cli.OpenConfiguredStore), or warn and run without one. A missing file
isn't created, and a database needing migration says to run job-search
migrate.
Fixed upsertOpportunity, which never wrote the source column, so every
saved opportunity had an empty Source.
