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.

crates/aether-testing/integration/src/bin/gateway_tunnel_stream_baseline.rs

@@ -104,8 +104,13 @@ struct AcceptanceReport {
     reasons: Vec<String>,
 }
 
+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>> {
     init_test_runtime_for("gateway-tunnel-stream-baseline");
     let config = parse_args(std::env::args().skip(1).collect())?;
     let report = run_suite(&config).await?;