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/circuitbreaker/circuitbreaker.go

@@ -22,15 +22,6 @@ type Config struct {
 	Name             string       // 熔断器名称，用于日志
 }
 
-func DefaultConfig(name string) Config {
-	return Config{
-		FailureThreshold: 5,
-		SuccessThreshold: 3,
-		Timeout:          30 * time.Second,
-		Name:             name,
-	}
-}
-
 type Breaker struct {
 	cfg    Config
 	mu     sync.Mutex