feat: add whitelist register mode

src/main/java/com/openisle/model/Notification.java

@@ -45,6 +45,9 @@ public class Notification {
     @Column(name = "reaction_type")
     private ReactionType reactionType;
 
+    @Column(length = 1000)
+    private String content;
+
     @Column
     private Boolean approved;
 

src/main/java/com/openisle/model/NotificationType.java

@@ -27,5 +27,7 @@ public enum NotificationType {
     /** Someone unfollowed you */
     USER_UNFOLLOWED,
     /** A user you subscribe to created a post or comment */
-    USER_ACTIVITY
+    USER_ACTIVITY,
+    /** A user requested registration approval */
+    REGISTER_REQUEST
 }

src/main/java/com/openisle/model/RegisterMode.java

@@ -0,0 +1,9 @@
+package com.openisle.model;
+
+/**
+ * Application-wide user registration mode.
+ */
+public enum RegisterMode {
+    DIRECT,
+    WHITELIST
+}

src/main/java/com/openisle/model/User.java

@@ -42,6 +42,12 @@ public class User {
     @Column(length = 1000)
     private String introduction;
 
+    @Column(length = 1000)
+    private String registerReason;
+
+    @Column(nullable = false)
+    private boolean approved = true;
+
     @Enumerated(EnumType.STRING)
     @Column(nullable = false)
     private Role role = Role.USER;