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>

migrations/migrate.go

@@ -96,6 +96,8 @@ func Migrate() {
 		&models.VcardApiConfig{},
 		// Integración Coolify
 		&models.CoolifyConfig{},
+		// Historial de métricas del agente
+		&models.ServidorMetricasHistory{},
 	); err != nil {
 		log.Fatalf("Error during main migration: %v", err)
 	}