feat(auth): implement yggdrasil join endpoint

internal/api/auth_handler.go

@@ -49,3 +49,32 @@ func (h *AuthHandler) Authenticate(w http.ResponseWriter, r *http.Request) {
 	w.WriteHeader(http.StatusOK)
 	json.NewEncoder(w).Encode(response)
 }
+
+func (h *AuthHandler) Join(w http.ResponseWriter, r *http.Request) {
+	var req models.JoinRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		http.Error(w, "Invalid request body", http.StatusBadRequest)
+		return
+	}
+
+	err := h.Service.ValidateJoinRequest(r.Context(), req)
+	if err != nil {
+		// Yggdrasil ожидает 403 Forbidden при невалидной сессии
+		if errors.Is(err, core.ErrInvalidCredentials) {
+			w.Header().Set("Content-Type", "application/json")
+			w.WriteHeader(http.StatusForbidden)
+			json.NewEncoder(w).Encode(YggdrasilError{
+				Error:        "ForbiddenOperationException",
+				ErrorMessage: "Invalid token.",
+			})
+			return
+		}
+
+		log.Printf("internal server error during join: %v", err)
+		http.Error(w, "Internal server error", http.StatusInternalServerError)
+		return
+	}
+
+	// В случае успеха возвращаем пустой ответ со статусом 204
+	w.WriteHeader(http.StatusNoContent)
+}