Added database.change_batch_retention, deleting old change batches

Each save that records a batch also deletes, in the same
transaction, the batches older than database.change_batch_retention
(30 days by default; 0 or less keeps them for ever), with their
records of opportunities. A client still holding a deleted batch
gets 410 Gone from ?changed_after= and reloads the whole list, as
for any unknown batch.

internal/cli/store.go

@@ -38,7 +38,11 @@ func OpenConfiguredStore(
 		return nil, fmt.Errorf("database %s: %w", file, err)
 	}
 
-	opportunityStore, err := store.Open(ctx, file)
+	opportunityStore, err := store.Open(
+		ctx,
+		file,
+		store.WithChangeBatchRetention(conf.Database.ChangeBatchRetention),
+	)
 	if errors.Is(err, store.ErrNeedsMigration) {
 		return nil, fmt.Errorf(
 			"database %s: %w: run `job-search migrate`", file, err,