feat: add Prometheus metrics shim for Indiekit process monitoring

Preloads metrics-shim.cjs via `node --require` into the Indiekit process
so heap, GC, event loop lag, CPU and handle metrics are exposed at
:9209/metrics for Prometheus scraping. Uses prom-client collectDefaultMetrics.

- Add metrics-shim.cjs (prom-client HTTP server, port 9209)
- Add prom-client ^15.1.3 to dependencies
- Wire --require ./metrics-shim.cjs into start.example.sh and npm serve script
- Grafana: NodeJS Application Dashboard (11159) at console.giersig.eu

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

metrics-shim.cjs

@@ -0,0 +1,45 @@
+'use strict';
+// Prometheus metrics shim — preloaded into the Indiekit process via:
+//   node --require ./metrics-shim.cjs <indiekit cli>
+// Because it runs inside the same process, all metrics (heap, GC, event
+// loop lag, CPU, open handles) reflect the actual Indiekit application.
+//
+// Grafana dashboards: NodeJS Application Dashboard (11159) at
+//   https://console.giersig.eu
+//
+// Config via environment variables (set in .env):
+//   METRICS_BIND_HOST  — defaults to 0.0.0.0
+//   METRICS_PORT       — defaults to 9209
+
+const http = require('http');
+const client = require('prom-client');
+
+const register = new client.Registry();
+client.collectDefaultMetrics({ register });
+
+const host = process.env.METRICS_BIND_HOST || '0.0.0.0';
+const port = parseInt(process.env.METRICS_PORT || '9209', 10);
+
+const server = http.createServer(async (req, res) => {
+  if (req.url === '/metrics') {
+    try {
+      const metrics = await register.metrics();
+      res.setHeader('Content-Type', register.contentType);
+      res.end(metrics);
+    } catch (err) {
+      res.writeHead(500);
+      res.end(String(err));
+    }
+  } else {
+    res.writeHead(404);
+    res.end('Not found\n');
+  }
+});
+
+server.listen(port, host, () => {
+  process.stderr.write(`[metrics] Prometheus metrics at http://${host}:${port}/metrics\n`);
+});
+
+server.on('error', (err) => {
+  process.stderr.write(`[metrics] Server error: ${err.message}\n`);
+});