feat(moderation): add manual content review system with three-tier AI moderation

Add admin comment moderation endpoints (single and batch) and introduce a three-tier moderation system (pass/review/block) for content flagged by AI. Content with unclear violations is now held for manual review instead of being auto-rejected. Deleted the legacy audit_service.go in favor of the unified hook-based moderation system.

internal/model/comment.go

@@ -33,7 +33,10 @@ type Comment struct {
 	Replies []*Comment `json:"-" gorm:"-"` // 子回复（手动加载，非 GORM 关联）
 
 	// 审核状态
-	Status CommentStatus `json:"status" gorm:"type:varchar(20);default:published;index:idx_comments_post_parent_status_created,priority:3;index:idx_comments_root_status_created,priority:2"`
+	Status       CommentStatus `json:"status" gorm:"type:varchar(20);default:published;index:idx_comments_post_parent_status_created,priority:3;index:idx_comments_root_status_created,priority:2"`
+	ReviewedAt   *time.Time    `json:"reviewed_at" gorm:"type:timestamp"`
+	ReviewedBy   string        `json:"reviewed_by" gorm:"type:varchar(50)"`
+	RejectReason string        `json:"reject_reason" gorm:"type:varchar(500)"`
 
 	// 统计
 	LikesCount   int `json:"likes_count" gorm:"default:0"`