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/cli/serve/command.go

@@ -29,9 +29,10 @@ func NewCommand(opts *cli.Options) *cobra.Command {
 		Short: "Serve the REST API (/api) and WebSocket (/ws), and fetch from sources",
 		Long: `Serve the REST API under /api and the WebSocket endpoint under /ws.
 
-It also runs a worker for every enabled source, as fetcher service does. A
-source that stops with an error stops the server too, and the server
-stopping stops the sources.`,
+It also runs a worker for every enabled source, as fetcher service does,
+saving what they fetch to database.file if it is set. A source that stops
+with an error stops the server too, and the server stopping stops the
+sources.`,
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
 			slog.Debug("command persistent pre-run", "command", "serve")
 		},
@@ -118,7 +119,23 @@ stopping stops the sources.`,
 			runCtx, cancelRun := context.WithCancel(ctx)
 			defer cancelRun()
 
-			sourceFetcher := fetcher.New(&conf.Sources)
+			var fetcherOptions []fetcher.Option
+			opportunityStore, openErr := cli.OpenConfiguredStore(ctx, conf)
+			switch {
+			case errors.Is(openErr, cli.ErrNoDatabase):
+				slog.Warn("not storing fetched opportunities", "reason", openErr)
+			case openErr != nil:
+				return openErr
+			default:
+				defer func() {
+					err = errors.Join(err, opportunityStore.Close())
+				}()
+				fetcherOptions = append(
+					fetcherOptions, fetcher.WithStore(opportunityStore),
+				)
+			}
+
+			sourceFetcher := fetcher.New(&conf.Sources, fetcherOptions...)
 			fetcherDone := make(chan error, 1)
 			go func() {
 				fetchErr := sourceFetcher.Run(runCtx, time.Time{})