feat(admin): 独立后台壳、站点分析与聊天监管，公开站拆出主题切换与访问埋点

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/model/db.go

@@ -37,7 +37,7 @@ func InitDB(dsn string) error {
 
 	if err := db.AutoMigrate(
 		&User{}, &Board{}, &Post{}, &Comment{}, &CommentEditHistory{}, &RefreshToken{}, &Like{}, &Notification{}, &Checkin{},
-		&Announcement{}, &SiteSetting{}, &Attachment{}, &UserBoard{}, &LoginLog{},
+		&Announcement{}, &SiteSetting{}, &SiteDailyStats{}, &SiteDailyVisitor{}, &Attachment{}, &UserBoard{}, &LoginLog{},
 		&ChatRoom{}, &ChatRoomMember{}, &ChatMessage{},
 		&PointLedger{}, &PostContentUnlock{}, &PostAttachment{}, &PostAttachmentUnlock{},
 		&PostPollVote{}, &PostLotteryEntry{},

backend/model/models.go

@@ -400,6 +400,21 @@ type SiteSetting struct {
 	UpdatedAt time.Time `json:"updated_at"`
 }
 
+// SiteDailyStats 站点日维度访问量（轻量 PV/UV）
+type SiteDailyStats struct {
+	Date       time.Time `gorm:"type:date;primaryKey" json:"date"`
+	PV         int64     `gorm:"not null;default:0" json:"pv"`
+	UV         int64     `gorm:"not null;default:0" json:"uv"`
+	LoggedInPV int64     `gorm:"not null;default:0" json:"logged_in_pv"`
+	UpdatedAt  time.Time `json:"updated_at"`
+}
+
+// SiteDailyVisitor 当日访客去重（vid 哈希）；插入成功才给 UV +1
+type SiteDailyVisitor struct {
+	Date    time.Time `gorm:"type:date;primaryKey" json:"date"`
+	VidHash string    `gorm:"size:64;primaryKey" json:"-"`
+}
+
 // 附件类型
 const (
 	AttachmentKindAvatar = "avatar" // 用户头像（正方形 WebP）