feat(agent): persist 7-day metrics history per server heartbeat

- New table servidor_metricas_history (cpu%, ram%, swap%, disco%, tcp, load1)
- Each heartbeat inserts a compact row (~60 bytes)
- GET /app/servidor/:id/metricas-history?horas=24 returns the data
- Daily cron at 3AM purges rows older than 7 days
- ~1.2 MB per server per 7 days at 30s interval

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/services/cron_service.go

@@ -48,6 +48,18 @@ func IniciarCron() {
 		return
 	}
 
+	// Purga de historial de métricas — cada noche a las 3 AM (retención 7 días)
+	if _, err := cronScheduler.AddFunc("0 3 * * *", func() {
+		if err := models.PurgarMetricasHistory(7); err != nil {
+			log.Printf("[CRON] Error purgando historial métricas: %v", err)
+		} else {
+			log.Println("[CRON] Historial de métricas purgado (>7 días)")
+		}
+	}); err != nil {
+		log.Printf("[CRON] Error registrando tarea purga_metricas: %v", err)
+		return
+	}
+
 	cronScheduler.Start()
 	log.Println("[CRON] Scheduler iniciado — vencimientos próximos 8AM, ya vencidos 9AM, Bold polling cada 15min, salud servidores cada 5min")
 }