fix(stammbaum): structured error codes in RelationshipController

getRelationshipBetween now throws DomainException with RELATIONSHIP_NOT_FOUND
instead of ResponseStatusException, so the frontend receives a typed error code.
Removed redundant validateRelationType() guard — RelationshipService.parseType()
already handles this with the same DomainException/VALIDATION_ERROR path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/test/java/org/raddatz/familienarchiv/relationship/RelationshipControllerTest.java

@@ -0,0 +1,57 @@
+package org.raddatz.familienarchiv.relationship;
+
+import org.junit.jupiter.api.Test;
+import org.raddatz.familienarchiv.config.SecurityConfig;
+import org.raddatz.familienarchiv.exception.ErrorCode;
+import org.raddatz.familienarchiv.security.PermissionAspect;
+import org.raddatz.familienarchiv.service.CustomUserDetailsService;
+import org.raddatz.familienarchiv.service.UserService;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.autoconfigure.aop.AopAutoConfiguration;
+import org.springframework.boot.webmvc.test.autoconfigure.WebMvcTest;
+import org.springframework.context.annotation.Import;
+import org.springframework.http.MediaType;
+import org.springframework.security.test.context.support.WithMockUser;
+import org.springframework.test.context.bean.override.mockito.MockitoBean;
+import org.springframework.test.web.servlet.MockMvc;
+
+import java.util.Optional;
+import java.util.UUID;
+
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.when;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
+
+@WebMvcTest(RelationshipController.class)
+@Import({SecurityConfig.class, PermissionAspect.class, AopAutoConfiguration.class})
+class RelationshipControllerTest {
+
+    @Autowired MockMvc mockMvc;
+
+    @MockitoBean RelationshipService relationshipService;
+    @MockitoBean UserService userService;
+    @MockitoBean CustomUserDetailsService customUserDetailsService;
+
+    private static final UUID PERSON_ID = UUID.randomUUID();
+    private static final UUID OTHER_ID  = UUID.randomUUID();
+
+    @Test
+    @WithMockUser(username = "testuser", authorities = {"READ_ALL"})
+    void getRelationshipBetween_returns404_with_RELATIONSHIP_NOT_FOUND_code_when_no_path() throws Exception {
+        when(relationshipService.getRelationshipBetween(any(), any())).thenReturn(Optional.empty());
+
+        mockMvc.perform(get("/api/persons/{aId}/relationship-to/{bId}", PERSON_ID, OTHER_ID))
+                .andExpect(status().isNotFound())
+                .andExpect(jsonPath("$.code").value(ErrorCode.RELATIONSHIP_NOT_FOUND.name()));
+    }
+
+    @Test
+    @WithMockUser(username = "testuser", authorities = {"READ_ALL"})
+    void addRelationship_returns403_for_user_with_READ_ALL_only() throws Exception {
+        mockMvc.perform(post("/api/persons/{id}/relationships", PERSON_ID)
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("{\"relatedPersonId\":\"" + OTHER_ID + "\",\"relationType\":\"PARENT_OF\"}"))
+                .andExpect(status().isForbidden());
+    }
+}