Fix: Resolve ApplicationContext failure in integration tests

- Added @MockBean for UserDetailsService to SecurityConfigIntegrationTest.
- This prevents the initial ApplicationContext load failure by providing a
  mock dependency for Spring Security, allowing all subsequent integration
  tests to run.

Author: rkonoplev

backend/src/test/java/com/example/phoebe/integration/config/SecurityConfigIntegrationTest.java

@@ -11,6 +11,8 @@
 import org.junit.jupiter.api.Test;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
+import org.springframework.boot.test.mock.mockito.MockBean;
+import org.springframework.security.core.userdetails.UserDetailsService;
 import org.springframework.security.crypto.password.PasswordEncoder;
 import org.springframework.test.annotation.DirtiesContext;
 import org.springframework.test.web.servlet.MockMvc;
@@ -49,6 +51,9 @@ class SecurityConfigIntegrationTest extends BaseIntegrationTest {
     @Autowired
     private NewsRepository newsRepository;
 
+    @MockBean
+    private UserDetailsService userDetailsService;
+
     private String adminAuthHeader;
     private String editorAuthHeader;