Made the fetcher save what it fetches, with fetcher.WithStore

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.

internal/store/store_test.go

@@ -52,6 +52,7 @@ func opportunity(
 		EndDate:           &end,
 		GrossValue:        125000.50,
 		HasDocuments:      true,
+		Source:            "FindATender",
 		Classifications: []types.ClassificationCode{
 			{
 				IsCPV:          true,
@@ -144,7 +145,8 @@ func TestSaveAndGetRoundTrip(t *testing.T) {
 		got.ProcurementMethod != in.ProcurementMethod || got.UUIDType != "ocid" ||
 		got.Status != "active" || got.Description != in.Description ||
 		!got.MultiLot || !got.SMESuitable || got.VCSESuitable ||
-		!got.HasDocuments || got.GrossValue != in.GrossValue {
+		!got.HasDocuments || got.GrossValue != in.GrossValue ||
+		got.Source != in.Source {
 		t.Errorf("opportunity fields differ:\n got %+v\nwant %+v", got, in)
 	}
 	if got.StartDate == nil || !got.StartDate.Equal(day(20)) ||