test(auth): login response never leaks the password field

Pin the @JsonProperty(WRITE_ONLY) invariant on AppUser.password. If the
annotation is ever dropped — or a new field aliases the hash — the CI run that
ships the regression flags it the next morning rather than waiting for a
security review. Addresses PR #612 / Nora concern (regression test).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/test/java/org/raddatz/familienarchiv/auth/AuthSessionControllerTest.java

@@ -100,6 +100,31 @@ class AuthSessionControllerTest {
         verify(sessionAuthenticationStrategy).onAuthentication(eq(auth), any(), any());
     }
 
+    @Test
+    void login_response_body_does_not_contain_password_field() throws Exception {
+        // Regression guard: AppUser.password is @JsonProperty(WRITE_ONLY). If anyone
+        // ever drops that annotation, this assertion catches the credential leak on
+        // the very next CI run.
+        UUID userId = UUID.randomUUID();
+        AppUser appUser = AppUser.builder()
+            .id(userId)
+            .email("leak@test.de")
+            .password("$2a$10$shouldnotappearinresponse")
+            .build();
+        Authentication auth = mock(Authentication.class);
+        when(authService.login(anyString(), anyString(), anyString(), anyString()))
+            .thenReturn(new LoginResult(appUser, auth));
+
+        mockMvc.perform(post("/api/auth/login")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"email\":\"leak@test.de\",\"password\":\"pass\"}"))
+            .andExpect(status().isOk())
+            .andExpect(jsonPath("$.password").doesNotExist())
+            .andExpect(jsonPath("$.pwd").doesNotExist())
+            .andExpect(content().string(org.hamcrest.Matchers.not(
+                org.hamcrest.Matchers.containsString("$2a$10$shouldnotappearinresponse"))));
+    }
+
     @Test
     void login_does_not_set_cookie_on_failure() throws Exception {
         when(authService.login(anyString(), anyString(), anyString(), anyString()))