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/repository/device.go

@@ -0,0 +1,87 @@
+package repository
+
+import (
+	"database/sql"
+	"pc-monitor-server/model"
+	"time"
+)
+
+type DeviceRepository struct {
+	db *sql.DB
+}
+
+func NewDeviceRepository(db *sql.DB) *DeviceRepository {
+	return &DeviceRepository{db: db}
+}
+
+func (r *DeviceRepository) Create(device *model.Device) error {
+	_, err := r.db.Exec(
+		`INSERT INTO devices (id, hostname, os, ip, registered_at, last_report_at, status)
+		 VALUES (?, ?, ?, ?, ?, ?, ?)`,
+		device.ID, device.Hostname, device.OS, device.IP,
+		device.RegisteredAt, device.LastReportAt, device.Status,
+	)
+	return err
+}
+
+func (r *DeviceRepository) Update(device *model.Device) error {
+	_, err := r.db.Exec(
+		`UPDATE devices SET hostname=?, os=?, ip=?, last_report_at=?, status=?
+		 WHERE id=?`,
+		device.Hostname, device.OS, device.IP,
+		device.LastReportAt, device.Status, device.ID,
+	)
+	return err
+}
+
+func (r *DeviceRepository) GetByID(id string) (*model.Device, error) {
+	device := &model.Device{}
+	err := r.db.QueryRow(
+		`SELECT id, hostname, os, ip, registered_at, last_report_at, status
+		 FROM devices WHERE id=?`, id,
+	).Scan(&device.ID, &device.Hostname, &device.OS, &device.IP,
+		&device.RegisteredAt, &device.LastReportAt, &device.Status)
+	if err != nil {
+		return nil, err
+	}
+	return device, nil
+}
+
+func (r *DeviceRepository) GetAll() ([]model.Device, error) {
+	rows, err := r.db.Query(
+		`SELECT id, hostname, os, ip, registered_at, last_report_at, status
+		 FROM devices ORDER BY last_report_at DESC`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var devices []model.Device
+	for rows.Next() {
+		var d model.Device
+		if err := rows.Scan(&d.ID, &d.Hostname, &d.OS, &d.IP,
+			&d.RegisteredAt, &d.LastReportAt, &d.Status); err != nil {
+			return nil, err
+		}
+		devices = append(devices, d)
+	}
+	return devices, nil
+}
+
+func (r *DeviceRepository) Delete(id string) error {
+	_, err := r.db.Exec("DELETE FROM devices WHERE id=?", id)
+	return err
+}
+
+func (r *DeviceRepository) UpdateStatus(id string, status string) error {
+	_, err := r.db.Exec("UPDATE devices SET status=? WHERE id=?", status, id)
+	return err
+}
+
+func (r *DeviceRepository) MarkOffline(threshold time.Duration) error {
+	cutoff := time.Now().Add(-threshold)
+	_, err := r.db.Exec(
+		`UPDATE devices SET status='offline'
+		 WHERE last_report_at < ? AND status='online'`, cutoff)
+	return err
+}