fix(admin): guard GET /api/users/{id} with @RequirePermission(ADMIN_USER)

Fixes IDOR: the endpoint was publicly accessible to any authenticated user.
Now requires ADMIN_USER permission, matching all other user management endpoints.

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

backend/src/test/java/org/raddatz/familienarchiv/controller/UserControllerTest.java

@@ -50,4 +50,29 @@ class UserControllerTest {
                 .andExpect(status().isOk())
                 .andExpect(jsonPath("$.username").value("anna"));
     }
+
+    // ─── GET /api/users/{id} ──────────────────────────────────────────────────
+
+    @Test
+    @WithMockUser(username = "reader")
+    void getUser_returns403_whenCallerLacksAdminUserPermission() throws Exception {
+        UUID id = UUID.randomUUID();
+        AppUser target = AppUser.builder().id(id).username("target").build();
+        when(userService.getById(id)).thenReturn(target);
+
+        mockMvc.perform(get("/api/users/" + id))
+                .andExpect(status().isForbidden());
+    }
+
+    @Test
+    @WithMockUser(username = "admin", authorities = {"ADMIN_USER"})
+    void getUser_returns200_whenCallerHasAdminUserPermission() throws Exception {
+        UUID id = UUID.randomUUID();
+        AppUser user = AppUser.builder().id(id).username("target").build();
+        when(userService.getById(id)).thenReturn(user);
+
+        mockMvc.perform(get("/api/users/" + id))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$.username").value("target"));
+    }
 }