feat(restore): add --parallel-dbs=-1 auto-detection based on CPU/RAM

- Add CalculateOptimalParallel() function to preflight.go
- Calculates optimal workers: min(RAM/3GB, CPU cores), capped at 16
- Reduces parallelism by 50% if memory pressure >80%
- Add -1 flag value for auto-detection mode
- Preflight summary now shows CPU cores and recommended parallel

cmd/restore.go

@@ -290,7 +290,7 @@ func init() {
 	restoreClusterCmd.Flags().BoolVar(&restoreForce, "force", false, "Skip safety checks and confirmations")
 	restoreClusterCmd.Flags().BoolVar(&restoreCleanCluster, "clean-cluster", false, "Drop all existing user databases before restore (disaster recovery)")
 	restoreClusterCmd.Flags().IntVar(&restoreJobs, "jobs", 0, "Number of parallel decompression jobs (0 = auto)")
-	restoreClusterCmd.Flags().IntVar(&restoreParallelDBs, "parallel-dbs", 0, "Number of databases to restore in parallel (0 = use config default, 1 = sequential)")
+	restoreClusterCmd.Flags().IntVar(&restoreParallelDBs, "parallel-dbs", 0, "Number of databases to restore in parallel (0 = use config default, 1 = sequential, -1 = auto-detect based on CPU/RAM)")
 	restoreClusterCmd.Flags().StringVar(&restoreWorkdir, "workdir", "", "Working directory for extraction (use when system disk is small, e.g. /mnt/storage/restore_tmp)")
 	restoreClusterCmd.Flags().BoolVar(&restoreVerbose, "verbose", false, "Show detailed restore progress")
 	restoreClusterCmd.Flags().BoolVar(&restoreNoProgress, "no-progress", false, "Disable progress indicators")
@@ -786,7 +786,12 @@ func runRestoreCluster(cmd *cobra.Command, args []string) error {
 	}
 
 	// Override cluster parallelism if --parallel-dbs is specified
-	if restoreParallelDBs > 0 {
+	if restoreParallelDBs == -1 {
+		// Auto-detect optimal parallelism based on system resources
+		autoParallel := restore.CalculateOptimalParallel()
+		cfg.ClusterParallelism = autoParallel
+		log.Info("Auto-detected optimal parallelism for database restores", "parallel_dbs", autoParallel, "mode", "auto")
+	} else if restoreParallelDBs > 0 {
 		cfg.ClusterParallelism = restoreParallelDBs
 		log.Info("Using custom parallelism for database restores", "parallel_dbs", restoreParallelDBs)
 	}