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>

rest/routes/user.go

@@ -81,6 +81,7 @@ func UserRoutes(app fiber.Router) {
 	protected.Get("/servidor-dashboard/:id", controllers.GetServidorDashboard)
 	protected.Get("/conx-ping/:id", controllers.PingConexion)
 	protected.Post("/servidor/:id/agent-token", controllers.GenerateAgentToken)
+	protected.Get("/servidor/:id/metricas-history", controllers.GetMetricasHistory)
 	protected.Post("/servidor/:id/sync-hostinger", controllers.SyncServidorFromHostinger)
 
 	// Rutas de proveedores de servidor