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/repository.go

@@ -0,0 +1,18 @@
+package repository
+
+import (
+	"database/sql"
+)
+
+// Repository bundles all tables' repos.
+type Repository struct {
+	Resources *Resources
+	Devices   *Devices
+}
+
+func NewRepository(conn *sql.DB) *Repository {
+	return &Repository{
+		Resources: &Resources{DB: conn},
+		Devices:   &Devices{DB: conn},
+	}
+}