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/store/store.go

@@ -716,6 +716,26 @@ func (s *Store) ListReleases(
 	return releases, nil
 }
 
+// LatestReleaseDate returns the date of the newest release stored for the
+// opportunities from source, or the zero time if none are stored, so a
+// fetcher can carry on from where it left off.
+func (s *Store) LatestReleaseDate(
+	ctx context.Context, source string,
+) (time.Time, error) {
+	var latest time.Time
+	err := s.db.GetContext(ctx, &latest, selectSourceLatestReleaseDate, source)
+	if errors.Is(err, sql.ErrNoRows) {
+		return time.Time{}, nil
+	}
+	if err != nil {
+		return time.Time{}, fmt.Errorf(
+			"reading the latest release date of %s: %w", source, err,
+		)
+	}
+
+	return latest, nil
+}
+
 // ReleaseExists reports whether a release is already stored, so a fetcher
 // can skip converting it.
 func (s *Store) ReleaseExists(