refactor: cleanup unused code and simplify internal packages

This commit performs a significant cleanup of the codebase by removing unused functions, methods, and entire files across various internal modules. This reduces technical debt and simplifies the project structure.

Key changes include:
- **cache**: Removed unused cache key generators and metrics snapshots.
- **dto**: Removed redundant converter functions and segment creation helpers.
- **middleware**: Deleted the unused `logger.go` middleware and simplified `ratelimit.go` and `casbin.go`.
- **model**: Removed unused ID helpers, database closing functions, and batch decryption logic.
- **pkg**: Cleaned up unused utility functions in `circuitbreaker`, `crypto`, `cursor`, `hook`, and `utils`.
- **service**: Deleted `account_cleanup_service.go` and removed unused helper functions in `log_cleanup_service.go` and `sensitive_service.go`.
- **repository**: Removed unused private loading methods in `comment_repo.go`.

internal/repository/comment_repo.go

@@ -241,20 +241,6 @@ func (r *commentRepository) GetByPostIDWithReplies(postID string, page, pageSize
 	return comments, total, nil
 }
 
-// loadFlatReplies 加载评论的所有回复（扁平化，所有层级都在同一层）
-func (r *commentRepository) loadFlatReplies(rootComment *model.Comment, limit int) {
-	var allReplies []*model.Comment
-
-	// 查询所有以该评论为根评论的回复（不包括顶级评论本身）
-	r.db.Where("root_id = ? AND status = ?", rootComment.ID, model.CommentStatusPublished).
-		Preload("User").
-		Order("created_at ASC").
-		Limit(limit).
-		Find(&allReplies)
-
-	rootComment.Replies = allReplies
-}
-
 // GetRepliesByRootID 根据根评论ID分页获取回复（扁平化）
 func (r *commentRepository) GetRepliesByRootID(rootID string, page, pageSize int) ([]*model.Comment, int64, error) {
 	var replies []*model.Comment