feat(web): add client-side telemetry with Web Vitals and dev metrics endpoint

apps/web/vite-plugin-metrics.ts

@@ -0,0 +1,42 @@
+import type { Plugin } from 'vite';
+
+/**
+ * Vite plugin that exposes a /metrics endpoint during development.
+ *
+ * The endpoint returns Prometheus‑text metrics collected in
+ * src/lib/telemetry.ts.
+ */
+export function metricsPlugin(): Plugin {
+  let telemetryModule: typeof import('./src/lib/telemetry') | null = null;
+
+  return {
+    name: 'zeavis-metrics',
+
+    configureServer(server) {
+      server.middlewares.use(async (req, res, next) => {
+        // Only handle GET /metrics
+        if (req.method !== 'GET' || !req.url?.startsWith('/metrics')) {
+          return next();
+        }
+
+        // Lazy‑load the telemetry module (ensures the app is bootstrapped first)
+        if (!telemetryModule) {
+          try {
+            telemetryModule = await server.ssrLoadModule('./src/lib/telemetry.ts') as typeof import('./src/lib/telemetry');
+          } catch {
+            // If the module isn't ready yet, return an empty body
+            res.statusCode = 200;
+            res.setHeader('Content-Type', 'text/plain; charset=utf-8');
+            res.end('# telemetry module not yet loaded\n');
+            return;
+          }
+        }
+
+        const body = telemetryModule.collectMetrics();
+        res.statusCode = 200;
+        res.setHeader('Content-Type', 'text/plain; charset=utf-8');
+        res.end(body);
+      });
+    },
+  };
+}