refactor: update module path from github.com/nexavpn/nexavpn/backend to nexavpn/backend

Update go.mod module declaration and all internal imports across the backend codebase to use simplified nexavpn/backend path instead of full GitHub URL.

backend/internal/auth/token.go

@@ -9,7 +9,7 @@ import (
 	"github.com/golang-jwt/jwt/v5"
 	"github.com/google/uuid"
 
-	"github.com/nexavpn/nexavpn/backend/internal/identity"
+	"nexavpn/backend/internal/identity"
 )
 
 func NewRefreshToken() (plain string, hashed string, err error) {