feat: init pc-monitor project

- Client: Go-based Windows hardware monitoring (CPU, GPU, memory, disk, network, power)
- Server: Go + Gin + SQLite backend with REST API
- Frontend: Vue 3 + Element Plus dashboard
- Docker deployment support
- Windows service installation script

server/service/metrics.go

@@ -0,0 +1,35 @@
+package service
+
+import (
+	"pc-monitor-server/model"
+	"pc-monitor-server/repository"
+	"time"
+)
+
+type MetricsService struct {
+	repo *repository.MetricsRepository
+}
+
+func NewMetricsService(repo *repository.MetricsRepository) *MetricsService {
+	return &MetricsService{repo: repo}
+}
+
+func (s *MetricsService) Save(m *model.Metrics) error {
+	return s.repo.Save(m)
+}
+
+func (s *MetricsService) GetLatest(deviceID string) (*model.Metrics, error) {
+	return s.repo.GetLatest(deviceID)
+}
+
+func (s *MetricsService) GetHistory(deviceID string, start, end time.Time, limit int) ([]model.Metrics, error) {
+	if limit <= 0 {
+		limit = 1000
+	}
+	return s.repo.GetHistory(deviceID, start, end, limit)
+}
+
+func (s *MetricsService) Cleanup(retentionDays int) error {
+	before := time.Now().AddDate(0, 0, -retentionDays)
+	return s.repo.Cleanup(before)
+}