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/handler.go

@@ -4,9 +4,9 @@ import (
 	"encoding/json"
 	"net/http"
 
-	"github.com/nexavpn/nexavpn/backend/internal/apiutil"
-	"github.com/nexavpn/nexavpn/backend/internal/audit"
-	"github.com/nexavpn/nexavpn/backend/internal/requestctx"
+	"nexavpn/backend/internal/apiutil"
+	"nexavpn/backend/internal/audit"
+	"nexavpn/backend/internal/requestctx"
 )
 
 type Handler struct {

backend/internal/auth/service.go

@@ -7,7 +7,7 @@ import (
 
 	"github.com/google/uuid"
 
-	"github.com/nexavpn/nexavpn/backend/internal/identity"
+	"nexavpn/backend/internal/identity"
 )
 
 var ErrInvalidCredentials = errors.New("invalid credentials")

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) {