serve: graceful-shutdown bug — was exit-looping every 15s

The serve loop wrapped axum::serve in tokio::time::timeout(15s),
which caps the WHOLE serve future, not just the shutdown drain.
Net effect: skald-serve cleanly returned Ok after 15 seconds every
time, docker restart picked it up, container went through the
exit-loop. Made any long-running docker exec (like summarize, with
opus calls that take 60-180s) racy at best, dead at worst — the
embedded postgres got 'database system was not properly shut down'
every 15s on the dot.

Fix: move the 15s deadline INSIDE the shutdown future. axum::serve
runs forever; the shutdown future fires on SIGTERM/SIGINT, then
gives in-flight requests 15s, then forces exit. Container only
goes down on a real signal.

Same bug exists in cwho-panel (copy-pasted from there). Fixing
there in a separate commit.

skald/src/serve.rs

@@ -39,14 +39,23 @@ pub async fn run(database_url: &str, listen: &str) -> anyhow::Result<()> {
     let listener = tokio::net::TcpListener::bind(listen).await?;
     tracing::info!(listen, "api listening");
 
-    let serve = axum::serve(listener, router).with_graceful_shutdown(shutdown());
-    match tokio::time::timeout(Duration::from_secs(15), serve).await {
-        Ok(r) => r?,
-        Err(_) => tracing::warn!("graceful shutdown timed out after 15s — exiting anyway"),
-    }
+    // Run forever; exit only when SIGTERM/SIGINT signals the
+    // graceful-shutdown future. The shutdown future itself caps
+    // drain time so in-flight requests can't block exit forever.
+    axum::serve(listener, router)
+        .with_graceful_shutdown(shutdown_with_deadline())
+        .await?;
     Ok(())
 }
 
+async fn shutdown_with_deadline() {
+    shutdown().await;
+    tracing::info!("graceful drain starting (15s deadline)");
+    // 15s for in-flight requests to finish; then force-exit.
+    tokio::time::sleep(Duration::from_secs(15)).await;
+    tracing::warn!("graceful drain deadline elapsed — forcing exit");
+}
+
 async fn shutdown() {
     let ctrl_c = async {
         let _ = tokio::signal::ctrl_c().await;