Stage 6: alert statistics endpoints

- GET /api/stats/alerts       — total/firing/resolved counts
- GET /api/stats/alerts/top   — most frequent alert names (?limit, default 10)
- GET /api/stats/alerts/by-hour — counts for all 24 hours (zeros filled in)
- GET /api/stats/alerts/by-day  — counts for all 7 days with names (zeros filled in)
- All endpoints accept optional ?from/?to (YYYY-MM-DD) to filter by received_at

internal/api/router.go

@@ -43,6 +43,11 @@ func NewRouter(db *sql.DB) http.Handler {
 		r.Get("/api/schedule/current", handleCurrentSchedule(db)) // must be before /{id}
 		r.Get("/api/schedule", handleListSchedule(db))
 		r.Delete("/api/schedule/{id}", handleDeleteSchedule(db))
+
+		r.Get("/api/stats/alerts", handleStatsAlerts(db))
+		r.Get("/api/stats/alerts/top", handleStatsTop(db))
+		r.Get("/api/stats/alerts/by-hour", handleStatsByHour(db))
+		r.Get("/api/stats/alerts/by-day", handleStatsByDay(db))
 	})
 
 	return r