feat: add whitelist register mode

src/main/java/com/openisle/service/RegisterModeService.java

@@ -0,0 +1,25 @@
+package com.openisle.service;
+
+import com.openisle.model.RegisterMode;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.stereotype.Service;
+
+/**
+ * Holds current registration mode. Configurable at runtime.
+ */
+@Service
+public class RegisterModeService {
+    private RegisterMode registerMode;
+
+    public RegisterModeService(@Value("${app.register.mode:WHITELIST}") RegisterMode registerMode) {
+        this.registerMode = registerMode;
+    }
+
+    public RegisterMode getRegisterMode() {
+        return registerMode;
+    }
+
+    public void setRegisterMode(RegisterMode mode) {
+        this.registerMode = mode;
+    }
+}