feat(08-01): add Prometheus /metrics endpoint with prom-client

- Install prom-client library for Prometheus metrics
- Create src/lib/server/metrics.ts with default Node.js process metrics
- Add /metrics endpoint that returns Prometheus-format text
- Exposes CPU, memory, heap, event loop metrics

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/lib/server/metrics.ts

@@ -0,0 +1,7 @@
+import { Registry, collectDefaultMetrics } from 'prom-client';
+
+// Create a custom registry for metrics
+export const registry = new Registry();
+
+// Collect default Node.js process metrics (CPU, memory, event loop, etc.)
+collectDefaultMetrics({ register: registry });

src/routes/metrics/+server.ts

@@ -0,0 +1,22 @@
+import type { RequestHandler } from './$types';
+import { registry } from '$lib/server/metrics';
+
+export const GET: RequestHandler = async () => {
+	try {
+		const metrics = await registry.metrics();
+
+		return new Response(metrics, {
+			status: 200,
+			headers: {
+				'Content-Type': registry.contentType
+			}
+		});
+	} catch (error) {
+		console.error('Metrics collection failed:', error);
+
+		return new Response('Metrics unavailable', {
+			status: 500,
+			headers: { 'Content-Type': 'text/plain' }
+		});
+	}
+};