feat(multiplayer): add down migrations and a rollback runner

Add migrations.Rollback(ctx, db, dir, steps): reverses the N most
recently applied migrations, newest first, each in its own committed
transaction under the same advisory lock Apply uses. Down SQL lives in
migrations/down/<version>.sql (a subdirectory, so Apply's *.sql glob
over the main directory is untouched); a missing down file for a
migration being rolled back is a hard error rather than a silent
partial reversal. Wire it into cmd/migrate as --rollback=N.

Add down files for all six existing migrations, each dropping objects
in FK-safe reverse dependency order.

Adversarial review: could not run the new integration test
(TestPostgreSQLMigrationsRollBackAndReapplyCleanly, gated behind
COSMIC_CLASH_POSTGRES_DSN / scripts/run_postgres_integration.sh)
against a real database in this sandbox - Docker Desktop's own
overlayfs ran out of space pulling postgres:17-alpine, unrelated to
this change. Verified instead by hand-tracing every DROP against its
forward migration's FK graph, confirming Apply's directory glob does
not pick up the down/ subdirectory, and a clean go build/vet/test
-tags integration. Worth an explicit real run before this is trusted
in CI.

server/migrations/runner_test.go

@@ -13,3 +13,12 @@ func TestApplyRejectsMissingDatabaseOrDirectory(t *testing.T) {
 		t.Fatal("empty directory accepted")
 	}
 }
+
+func TestRollbackRejectsMissingDatabaseDirectoryOrSteps(t *testing.T) {
+	if err := Rollback(context.Background(), nil, ".", 1); err == nil {
+		t.Fatal("nil database accepted")
+	}
+	if err := Rollback(context.Background(), nil, "", 1); err == nil {
+		t.Fatal("empty directory accepted")
+	}
+}