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)

server.v

@@ -15,9 +15,10 @@ pub struct Stats {
 pub mut:
 	total_notifications int
 	errors              int
+	http_polls          int // v0.5: total HTTP poll checks
 	by_source           map[string]int
+	start_time          time.Time
 }
-
 pub struct App {
 pub mut:
 	cfg              Config
@@ -190,6 +191,9 @@ pub fn (mut app App) handle(req http.Request) http.Response {
 			if req.url == '/api/silence' {
 				return app.silence_status_response()
 			}
+			if req.url == '/metrics' {
+				return app.metrics_response()
+			}
 		}
 		.post {
 			if req.url.starts_with('/api/silence') {
@@ -409,6 +413,7 @@ fn (mut app App) http_poll_loop() {
 		}
 		for src in app.cfg.sources.http_poll {
 			for check in src.checks {
+				app.stats.http_polls++
 				app.run_http_check(check)
 			}
 		}