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/pkg/cursor/pagination.go

@@ -99,15 +99,6 @@ type PageResponse struct {
 	HasMore    bool   `json:"has_more"`
 }
 
-// NewPageResponse 创建分页响应
-func NewPageResponse(items any, nextCursor, prevCursor string, hasMore bool) *PageResponse {
-	return &PageResponse{
-		Items:      items,
-		NextCursor: nextCursor,
-		PrevCursor: prevCursor,
-		HasMore:    hasMore,
-	}
-}
 
 // CursorPageResult 游标分页结果（泛型版本，供内部使用）
 type CursorPageResult[T any] struct {