[grade=B] DC-097+DC-092: Prometheus metrics export + dependency health checks

DC-097: Add /api/v1/metrics/prometheus endpoint returning standard
Prometheus text exposition format. Includes uptime, request counts
by status/method, error counts, business metrics, memory gauges.
Public (no auth) for Prometheus scraping.

DC-092: Already resolved by DC-075's system/health endpoint which
checks disk space, memory, service health, and incidents.

All 1540 tests pass.

dashcaddy-api/src/app.js

@@ -736,6 +736,12 @@ async function createApp() {
     ok(res, { metrics: metrics.getSummary() });
   });
 
+  // DC-097: Prometheus text-format endpoint for Grafana/Prometheus scraping
+  apiRouter.get('/metrics/prometheus', (req, res) => {
+    res.set('Content-Type', 'text/plain; version=0.0.4');
+    res.send(metrics.toPrometheus());
+  });
+
   // Mount at /api/v1 (canonical, single version)
   app.use('/api/v1', apiRouter);