feat: add whitelist register mode

src/main/java/com/openisle/controller/AdminUserController.java

@@ -0,0 +1,36 @@
+package com.openisle.controller;
+
+import com.openisle.model.User;
+import com.openisle.service.EmailSender;
+import com.openisle.repository.UserRepository;
+import lombok.RequiredArgsConstructor;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.*;
+
+@RestController
+@RequestMapping("/api/admin/users")
+@RequiredArgsConstructor
+public class AdminUserController {
+    private final UserRepository userRepository;
+    private final EmailSender emailSender;
+
+    @PostMapping("/{id}/approve")
+    public ResponseEntity<?> approve(@PathVariable Long id) {
+        User user = userRepository.findById(id).orElseThrow();
+        user.setApproved(true);
+        userRepository.save(user);
+        emailSender.sendEmail(user.getEmail(), "Registration Approved",
+                "Your account has been approved. Visit: https://www.open-isle.com");
+        return ResponseEntity.ok().build();
+    }
+
+    @PostMapping("/{id}/reject")
+    public ResponseEntity<?> reject(@PathVariable Long id) {
+        User user = userRepository.findById(id).orElseThrow();
+        user.setApproved(false);
+        userRepository.save(user);
+        emailSender.sendEmail(user.getEmail(), "Registration Rejected",
+                "Your account request was rejected. Visit: https://www.open-isle.com");
+        return ResponseEntity.ok().build();
+    }
+}