fix: resolve ERR_INSUFFICIENT_RESOURCES connection exhaustion

- Add concurrency limiter (max 4 GET requests) to API layer, leaving
  slots for SSE and health checks. Write ops bypass the limiter.
- Add AbortController to ContainerStats, project detail page, and
  dashboard to cancel in-flight requests on navigation/unmount.
- Move global SSE connection from layout to events page (only consumer).
- Add 30s heartbeat to SSE endpoint to detect zombie connections.
- Serialize dashboard project fetches to avoid parallel burst.
- Rebuild frontend in dev-server.sh so go:embed stays in sync.

web/src/lib/components/ContainerStats.svelte

@@ -18,24 +18,19 @@
 	let error = $state(false);
 
 	$effect(() => {
-		let cancelled = false;
-		let inflight = false;
+		let controller = new AbortController();
 
 		async function load() {
-			if (inflight) return; // Skip if previous request still pending.
-			inflight = true;
+			// Abort any previous in-flight request before starting a new one.
+			controller.abort();
+			controller = new AbortController();
 			try {
-				const result = await api.fetchContainerStats(projectId, stageId, instanceId);
-				if (!cancelled) {
-					stats = result;
-					error = false;
-				}
-			} catch {
-				if (!cancelled) {
-					error = true;
-				}
-			} finally {
-				inflight = false;
+				const result = await api.fetchContainerStats(projectId, stageId, instanceId, controller.signal);
+				stats = result;
+				error = false;
+			} catch (e) {
+				if (e instanceof DOMException && e.name === 'AbortError') return;
+				error = true;
 			}
 		}
 
@@ -45,7 +40,7 @@
 		const interval = setInterval(load, 30_000);
 
 		return () => {
-			cancelled = true;
+			controller.abort();
 			clearInterval(interval);
 		};
 	});