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/cache/metrics.go

@@ -14,25 +14,6 @@ type cacheMetrics struct {
 // metrics 全局缓存指标实例
 var metrics cacheMetrics
 
-// MetricsSnapshot 指标快照
-type MetricsSnapshot struct {
-	Hit         int64
-	Miss        int64
-	DecodeError int64
-	SetError    int64
-	Invalidate  int64
-}
-
-// GetMetricsSnapshot 获取当前指标快照
-func GetMetricsSnapshot() MetricsSnapshot {
-	return MetricsSnapshot{
-		Hit:         metrics.hit.Load(),
-		Miss:        metrics.miss.Load(),
-		DecodeError: metrics.decodeError.Load(),
-		SetError:    metrics.setError.Load(),
-		Invalidate:  metrics.invalidate.Load(),
-	}
-}
 
 // recordHit 记录缓存命中
 func recordHit() {