feat: Prometheus /metrics endpoint — Phase 5 -> 6/8

- metrics.v: GET /metrics au format OpenMetrics (gauges + counters)
- server.v: route /metrics, Stats.http_polls counter, increment in poll loop
- metrics_test.v: test du endpoint metrics
- ROADMAP: Phase 5 6/8 (Prometheus done)

metrics_test.v

@@ -0,0 +1,28 @@
+module main
+
+fn test_metrics_keys() {
+	mut app := App{
+		logger: new_logger(.info, false)
+		cfg: Config{
+			filters: FilterConfig{}
+			outgoing: OutgoingConfig{}
+			state: StateConfig{}
+		}
+		poll_state: map[string]bool{}
+	}
+	app.poll_state['http://example.com'] = true
+	app.poll_state['http://down.com'] = false
+	app.stats.total_notifications = 42
+	app.stats.errors = 3
+	app.stats.by_source['docker'] = 10
+
+	resp := app.metrics_response()
+	body := resp.body
+
+	assert body.contains('ntfy_bridge_uptime_seconds')
+	assert body.contains('ntfy_bridge_notifications_total 42')
+	assert body.contains('ntfy_bridge_errors_total 3')
+	assert body.contains('ntfy_bridge_silence_active')
+	assert body.contains('ntfy_bridge_http_polls_total')
+	assert body.contains('# EOF')
+}