feat: 帖子收藏与用户关注

- 新增收藏功能：favorite 服务与 handler、FavoriteButton 组件
- 新增关注功能：follow 服务与 handler、FollowButton 组件
- 用户主页适配关注/粉丝与收藏展示
- flash 消息工具、广告与侧边栏组件微调

backend/model/db.go

@@ -57,7 +57,7 @@ func InitDB(dsn string) error {
 
 	if err := db.AutoMigrate(
 		&TemporaryUpload{}, &ModuleConfig{}, &SettingsAudit{}, &ActionCounter{}, &MailTask{}, &EmailChallenge{}, &StoredObject{},
-		&User{}, &Board{}, &Post{}, &PostEditHistory{}, &Comment{}, &CommentEditHistory{}, &RefreshToken{}, &Like{}, &Notification{}, &Checkin{},
+		&User{}, &Board{}, &Post{}, &PostEditHistory{}, &Comment{}, &CommentEditHistory{}, &RefreshToken{}, &Like{}, &Favorite{}, &Follow{}, &Notification{}, &Checkin{},
 		&Announcement{}, &SitePage{}, &SiteSetting{}, &SiteDailyStats{}, &SiteDailyVisitor{}, &VisitEvent{}, &Attachment{}, &UserBoard{}, &LoginLog{},
 		&ChatRoom{}, &ChatRoomMember{}, &ChatMessage{},
 		&PointLedger{}, &PostContentUnlock{}, &PostAttachment{}, &PostAttachmentUnlock{},

backend/model/models.go

@@ -432,6 +432,22 @@ type Like struct {
 	CreatedAt time.Time `json:"created_at"`
 }
 
+// Favorite 收藏记录（联合唯一索引防止重复收藏；无计数列，按钮仅表示状态）
+type Favorite struct {
+	ID        uint      `gorm:"primaryKey" json:"id"`
+	PostID    uint      `gorm:"uniqueIndex:idx_fav_post_user;not null" json:"post_id"`
+	UserID    uint      `gorm:"uniqueIndex:idx_fav_post_user;not null" json:"user_id"`
+	CreatedAt time.Time `json:"created_at"`
+}
+
+// Follow 关注关系（FollowerID 关注了 FollowingID；联合唯一防止重复关注）
+type Follow struct {
+	ID          uint      `gorm:"primaryKey" json:"id"`
+	FollowerID  uint      `gorm:"uniqueIndex:idx_follow_pair,priority:1;index;not null" json:"follower_id"`
+	FollowingID uint      `gorm:"uniqueIndex:idx_follow_pair,priority:2;index;not null" json:"following_id"`
+	CreatedAt   time.Time `json:"created_at"`
+}
+
 // Checkin 每日签到记录（用户+日期联合唯一，防止重复签到）
 type Checkin struct {
 	ID          uint      `gorm:"primaryKey" json:"id"`
@@ -453,6 +469,7 @@ const (
 	NotificationTypeBadge         = "badge"          // 获得管理员颁发的徽章
 	NotificationTypeDeleted       = "deleted"        // 帖子被管理员删除
 	NotificationTypeNecroReply    = "necro_reply"    // 回复旧帖提醒（系统，ActorID=0）
+	NotificationTypeFollow        = "follow"         // 关注了你
 )
 
 // Notification 站内通知