feat: make global default timeout 0 (infinite) instead of required 30s

DefaultTimeoutSeconds now means "no timeout" when 0/empty, and that is
the new default, rather than an invalid config forcing a positive
value. runner.RunJob avoids context.WithTimeout with a zero duration
(which would expire immediately) and instead runs on a plain
cancelable context when no timeout is configured. Per-job
TimeoutSeconds inherit semantics are unchanged.

src/app/run_test.go

@@ -633,4 +633,9 @@ func TestEffectiveTimeout(t *testing.T) {
 	if got, want := svc.effectiveTimeout(own), 5*time.Second; got != want {
 		t.Errorf("per-job timeout = %s, want %s", got, want)
 	}
+
+	svc.store.Config.DefaultTimeoutSeconds = 0
+	if got, want := svc.effectiveTimeout(inherit), time.Duration(0); got != want {
+		t.Errorf("inherited timeout with no global default = %s, want %s (no timeout)", got, want)
+	}
 }