feat(api): CRUD files/folders + upload multipart réels (scope owner, soft-delete, conteurs d'erreurs)

- repository.Resources (insert/list/get/soft-delete scoping owner_id, UNIQUE(parent_id,name) → NAME_CONFLICT) + Devices.Upsert au register
- service.Resources : DTOs {id,name,size,mimeType,folderId,createdAt,updatedAt}, upload → UPLOAD_DIR/<device>/<id>.<ext> (nettoyage si métadonnée échoue)
- handlers : files list/get/delete/upload + folders racines ; routes consolidées dans handlers.RegisterRoutes
- dbtest package : test DB jetée par repo (open+reset+migrate, skip si PG down) ; tests repository + handlers end-to-end (register→token→CRUD, scoping cross-device, FILE_TOO_LARGE, NAME_CONFLICT)
- docs: /devices persiste le device, layout UPLOAD_DIR par device, codes NAME_CONFLICT/SERVICE_UNAVAILABLE

backend/repository/devices.go

@@ -0,0 +1,31 @@
+package repository
+
+import (
+	"database/sql"
+)
+
+// Devices persists registered devices (owner scope for every resource row).
+type Devices struct {
+	DB *sql.DB
+}
+
+// Upsert registers the device if absent and refreshes last_seen_at.
+func (d *Devices) Upsert(deviceID string) error {
+	_, err := d.DB.Exec(
+		`INSERT INTO devices (device_id, registered_at, last_seen_at)
+		 VALUES ($1, NOW(), NOW())
+		 ON CONFLICT (device_id) DO UPDATE SET last_seen_at = NOW()`,
+		deviceID,
+	)
+	return err
+}
+
+// Exists reports whether a device has been registered.
+func (d *Devices) Exists(deviceID string) (bool, error) {
+	var exists int
+	err := d.DB.QueryRow(`SELECT 1 FROM devices WHERE device_id = $1`, deviceID).Scan(&exists)
+	if err == sql.ErrNoRows {
+		return false, nil
+	}
+	return err == nil, err
+}