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 });