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/middleware/casbin.go

@@ -66,23 +66,6 @@ func CasbinAuth(casbinService service.CasbinService) gin.HandlerFunc {
 	}
 }
 
-// OptionalCasbinAuth 可选的 Casbin 权限检查
-// 不阻止请求，但会设置权限标志
-func OptionalCasbinAuth(casbinService service.CasbinService) gin.HandlerFunc {
-	return func(c *gin.Context) {
-		userID, exists := c.Get("user_id")
-		if exists {
-			path := c.Request.URL.Path
-			method := c.Request.Method
-			allowed, err := casbinService.EnforceForUser(c.Request.Context(), userID.(string), path, method)
-			if err == nil {
-				c.Set("permission_granted", allowed)
-			}
-		}
-		c.Next()
-	}
-}
-
 // RequireRole 要求特定角色的中间件
 func RequireRole(casbinService service.CasbinService, requiredRoles ...string) gin.HandlerFunc {
 	return func(c *gin.Context) {