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/fetcher/worker_findatender.go

@@ -13,11 +13,12 @@ import (
 )
 
 // runFindATender polls Find a Tender every configured interval until ctx
-// is cancelled.
+// is cancelled, passing what each run fetches to save.
 func runFindATender(
 	ctx context.Context,
 	logger *slog.Logger,
 	conf *config.JSCSourceFindATender,
+	save saveFunc,
 	since time.Time,
 ) error {
 	if conf.APIRoot == "" {
@@ -29,7 +30,11 @@ func runFindATender(
 		Name:     "FindATender",
 		Interval: conf.Interval,
 		Task: func(ctx context.Context, since time.Time) error {
-			return fetchFindATender(ctx, logger, conf, since)
+			opportunities, err := fetchFindATender(ctx, logger, conf, since)
+			if err != nil {
+				return err
+			}
+			return save(ctx, logger, opportunities)
 		},
 	}
 	return worker.Run(ctx, since)
@@ -43,7 +48,7 @@ func fetchFindATender(
 	logger *slog.Logger,
 	conf *config.JSCSourceFindATender,
 	since time.Time,
-) error {
+) ([]*types.Opportunity, error) {
 	opportunities, err := util.AwaitTask(
 		ctx,
 		func() ([]*types.Opportunity, error) {
@@ -55,13 +60,13 @@ func fetchFindATender(
 		},
 	)
 	if errors.Is(err, findatender.ErrNoReleases) {
-		return ErrNoNewReleases
+		return nil, ErrNoNewReleases
 	}
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	tagged(logger).Info("fetched opportunities", "count", len(opportunities))
 
-	return nil
+	return opportunities, nil
 }