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/main/java/org/raddatz/familienarchiv/controller/UserController.java

@@ -61,6 +61,7 @@ public class UserController {
     }
 
     @GetMapping("users/{id}")
+    @RequirePermission(Permission.ADMIN_USER)
     public ResponseEntity<AppUser> getUser(@PathVariable UUID id) {
         AppUser user = userService.getById(id);
         user.setPassword(null);