fix: harden concurrency limits and high-RPM runtime paths

Bound request, stream, queue, and shutdown resource lifetimes. Reduce scheduler and Redis hot-path work and isolate database maintenance. Include regression coverage, load probes, and concurrency audit results.

apps/aether-gateway/examples/execution_runtime_harness.rs

@@ -71,8 +71,13 @@ struct Args {
     distributed_request_command_timeout_ms: u64,
 }
 
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let _log_shutdown = aether_runtime::LogShutdownGuard::new();
+    run()
+}
+
 #[tokio::main]
-async fn main() -> Result<(), Box<dyn std::error::Error>> {
+async fn run() -> Result<(), Box<dyn std::error::Error>> {
     let _ = rustls::crypto::ring::default_provider().install_default();
 
     init_service_runtime(ServiceRuntimeConfig::new(