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/models/servidor.go

@@ -103,3 +103,49 @@ func DeleteServidor(servidor Servidor) error {
 	}
 	return nil
 }
+
+// ── Historial de métricas ─────────────────────────────────────────────────────
+
+type ServidorMetricasHistory struct {
+	ID          uint      `json:"id" gorm:"primaryKey;autoIncrement"`
+	ServidorID  uint      `json:"servidor_id" gorm:"column:servidor_id;index"`
+	CreatedAt   time.Time `json:"created_at" gorm:"column:created_at;index"`
+	CpuPct      float64   `json:"cpu_pct" gorm:"column:cpu_pct"`
+	RamPct      float64   `json:"ram_pct" gorm:"column:ram_pct"`
+	SwapPct     float64   `json:"swap_pct" gorm:"column:swap_pct"`
+	DiscoPct    float64   `json:"disco_pct" gorm:"column:disco_pct"`
+	TCPConns    int       `json:"tcp_conns" gorm:"column:tcp_conns"`
+	Load1       float64   `json:"load1" gorm:"column:load1"`
+}
+
+func (ServidorMetricasHistory) TableName() string { return "servidor_metricas_history" }
+
+func InsertMetricasHistory(servidorID uint, cpu, ram, swap, disco float64, tcp int, load1 float64) {
+	app.Http.Database.DB.Create(&ServidorMetricasHistory{
+		ServidorID: servidorID,
+		CreatedAt:  time.Now(),
+		CpuPct:     cpu,
+		RamPct:     ram,
+		SwapPct:    swap,
+		DiscoPct:   disco,
+		TCPConns:   tcp,
+		Load1:      load1,
+	})
+}
+
+func GetMetricasHistory(servidorID uint, horas int) ([]ServidorMetricasHistory, error) {
+	var items []ServidorMetricasHistory
+	desde := time.Now().Add(-time.Duration(horas) * time.Hour)
+	err := app.Http.Database.DB.
+		Where("servidor_id = ? AND created_at >= ?", servidorID, desde).
+		Order("created_at ASC").
+		Find(&items).Error
+	return items, err
+}
+
+func PurgarMetricasHistory(dias int) error {
+	corte := time.Now().AddDate(0, 0, -dias)
+	return app.Http.Database.DB.
+		Where("created_at < ?", corte).
+		Delete(&ServidorMetricasHistory{}).Error
+}