refactor: split backend into standalone server project

Remove embedded frontend and workflow editor assets, add standalone API deployment configuration, and retain the current backend service updates.

internal/services/message_service_test.go

@@ -187,6 +187,35 @@ func TestConversationCreateCreatesAIWelcomeMessage(t *testing.T) {
 	}
 }
 
+func TestConversationCreateAllowsHumanChannelWithoutAIAgent(t *testing.T) {
+	db := setupMessageWelcomeTestDB(t)
+
+	conversation, err := ConversationService.Create(welcomeTestExternalUser("human-channel-1"), 11, 0)
+	if err != nil {
+		t.Fatalf("create human channel conversation: %v", err)
+	}
+	if conversation == nil {
+		t.Fatal("expected conversation")
+	}
+	if conversation.AIAgentID != 0 {
+		t.Fatalf("ai agent id = %d, want 0", conversation.AIAgentID)
+	}
+	if conversation.ServiceMode != enums.IMConversationServiceModeHumanOnly {
+		t.Fatalf("service mode = %d, want human only", conversation.ServiceMode)
+	}
+	if conversation.Status != enums.IMConversationStatusPending {
+		t.Fatalf("status = %d, want pending", conversation.Status)
+	}
+
+	var messages []models.Message
+	if err := db.Where("conversation_id = ?", conversation.ID).Find(&messages).Error; err != nil {
+		t.Fatalf("find messages: %v", err)
+	}
+	if len(messages) != 1 || messages[0].Content != HandoffWaitingMessage {
+		t.Fatalf("unexpected human channel messages: %#v", messages)
+	}
+}
+
 func TestSendCustomerMessageStoresRequestIDOnMessageAndEvent(t *testing.T) {
 	db := setupMessageWelcomeTestDB(t)
 	aiAgent := createWelcomeTestAIAgent(t, db, "")