Made each source start from its newest stored release, with --since as an override

Without --since, fetcher service and job-search serve start each source
from the newest release stored for it (the new store.LatestReleaseDate,
by Source), so a restart carries on where it left off instead of fetching
the last day again; with nothing stored, the client's default still
applies. --since overrides it for every source. The Find a Tender source's
name is now a constant, as the Source its opportunities are stored with.

The README's guide to adding a source is brought up to date with the
current worker, start and config type names.

internal/fetcher/fetcher.go

@@ -99,6 +99,11 @@ func New(conf *config.JSCSources, opts ...Option) *Fetcher {
 // logged as they happen and returned joined; nil means a clean shutdown. Run
 // cancels the Fetcher's background context as it returns, so it can only be
 // called once.
+//
+// Every worker starts from since. A zero since starts each source from the
+// newest release stored for it (see WithStore and startFrom); with no store
+// or nothing stored, the source's client picks (Find a Tender's: the last
+// day).
 func (f *Fetcher) Run(ctx context.Context, since time.Time) error {
 	defer f.backgroundCancel()
 
@@ -114,14 +119,22 @@ func (f *Fetcher) Run(ctx context.Context, since time.Time) error {
 		errs []error
 	)
 
-	// start runs one source's worker until f.backgroundCtx is cancelled. Each
-	// worker is given logger untagged: it tags its own lines with tagged, and
-	// passes logger as it is to any client it creates.
-	start := func(name string, run func(ctx context.Context) error) {
+	// start runs one source's worker from where it starts (see startFrom)
+	// until f.backgroundCtx is cancelled. name is also the Source its
+	// opportunities are stored with. Each worker is given logger untagged: it
+	// tags its own lines with tagged, and passes logger as it is to any client
+	// it creates.
+	start := func(
+		name string, run func(ctx context.Context, since time.Time) error,
+	) {
 		wg.Add(1)
 		go func() {
 			defer wg.Done()
-			if err := run(f.backgroundCtx); err != nil {
+			sourceSince, err := f.startFrom(f.backgroundCtx, logger, name, since)
+			if err == nil {
+				err = run(f.backgroundCtx, sourceSince)
+			}
+			if err != nil {
 				tagged(logger).Error(
 					"source stopped, shutting down",
 					"source", name,
@@ -136,17 +149,17 @@ func (f *Fetcher) Run(ctx context.Context, since time.Time) error {
 	}
 
 	if f.conf.FindATender.Enabled {
-		start("FindATender", func(ctx context.Context) error {
+		start(findATenderSource, func(ctx context.Context, since time.Time) error {
 			return runFindATender(ctx, logger, &f.conf.FindATender, f.save, since)
 		})
 	}
 	if f.conf.FTSMock.Enabled {
-		start("FTSMock", func(ctx context.Context) error {
+		start(ftsMockSource, func(ctx context.Context, since time.Time) error {
 			return runFTSMock(ctx, logger, &f.conf.FTSMock, f.save, since)
 		})
 	}
 	if f.conf.Sleeper.Enabled {
-		start("Sleeper", func(ctx context.Context) error {
+		start("Sleeper", func(ctx context.Context, since time.Time) error {
 			return runSleeper(ctx, logger, &f.conf.Sleeper, since)
 		})
 	}
@@ -158,6 +171,34 @@ func (f *Fetcher) Run(ctx context.Context, since time.Time) error {
 	return errors.Join(errs...)
 }
 
+// startFrom returns the time source's worker starts from: since if it is
+// set, as an override. Otherwise it is the date of the newest release stored
+// for source, so a restart carries on where the last run left off, or the
+// zero time with no store or nothing stored yet, which leaves the start to
+// the source's client. Fetching from the newest release's date fetches it
+// again; the store skips it.
+func (f *Fetcher) startFrom(
+	ctx context.Context, logger *slog.Logger, source string, since time.Time,
+) (time.Time, error) {
+	if !since.IsZero() || f.store == nil {
+		return since, nil
+	}
+
+	latest, err := f.store.LatestReleaseDate(ctx, source)
+	if err != nil {
+		return time.Time{}, err
+	}
+	if !latest.IsZero() {
+		tagged(logger).Info(
+			"starting from the newest stored release",
+			"source", source,
+			"since", latest,
+		)
+	}
+
+	return latest, nil
+}
+
 // save saves opportunities to the store given WithStore, and logs what
 // changed. Without a store it does nothing. A failure fails the worker's run,
 // so its checkpoint isn't moved on and the next run fetches them again.