feat(platform): 日志管理 - 统一日志出口并屏蔽查询与 Agent 流式事件日志

新增 internal/logging，slog、标准库 log、GORM 与 Gin 访问日志共用同一 Handler，由 CANVAS_LOG_LEVEL / CANVAS_LOG_FORMAT 控制。info 级别不再输出成功的查询类请求、成功 SQL、message_end 等 Agent 流式事件和常规画布自动保存；4xx/5xx、慢请求、慢 SQL、SQL 失败与 panic 仍会记录，SQL 不内联参数值。

backend/internal/app/cloud_agent.go

@@ -5,7 +5,7 @@ import (
 	"encoding/hex"
 	"encoding/json"
 	"errors"
-	"log"
+	"log/slog"
 	"math"
 	"strings"
 	"time"
@@ -403,7 +403,7 @@ func (s *Service) CreateCloudAgentRun(userID string, req CloudAgentRequest, pare
 			return nil, err
 		}
 		if superseded {
-			log.Printf("agent run %s cannot resume after a contract change; continuing in a new turn", parentID)
+			slog.Info("agent run cannot resume after a contract change; continuing in a new turn", "run", parentID)
 		} else if !cloudAgentRunTerminal(parentRun.Status) || parentRun.CleanupPending {
 			return nil, kernel.NewAppError(409, "上一轮 Agent 尚未结束")
 		}