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/dto/message_converter.go

@@ -107,14 +107,6 @@ func ConvertMessagesToResponse(messages []*model.Message) []*MessageResponse {
 	return result
 }
 
-// ConvertConversationsToResponse 将Conversation列表转换为响应列表
-func ConvertConversationsToResponse(convs []*model.Conversation) []*ConversationResponse {
-	result := make([]*ConversationResponse, 0, len(convs))
-	for _, conv := range convs {
-		result = append(result, ConvertConversationToResponse(conv, nil, 0, nil, false, false))
-	}
-	return result
-}
 
 // ==================== PushRecord 转换 ====================