feat: add manual "转人工" (request human) action for conversations

- Backend: RequestHuman service method changes status from AI_SERVING to PENDING
- Backend: POST /api/dashboard/conversation/request-human endpoint
- Frontend: requestHumanConversation API client
- Frontend: "转人工" button in conversation list dropdown and detail dialog

internal/pkg/dto/request/conversation_request.go

@@ -34,6 +34,10 @@ type TransferConversationRequest struct {
 	Reason         string `json:"reason"`
 }
 
+type RequestHumanConversationRequest struct {
+	ConversationID int64 `json:"conversationId"`
+}
+
 type CloseConversationRequest struct {
 	ConversationID int64  `json:"conversationId"`
 	CloseReason    string `json:"closeReason"`