feat(restore): add weighted progress, pre-extraction disk check, parallel-dbs flag

Three high-value improvements for cluster restore:

1. Weighted progress by database size
   - Progress now shows percentage by data volume, not just count
   - Phase 3/3: Databases (2/7) - 45.2% by size
   - Gives more accurate ETA for clusters with varied DB sizes

2. Pre-extraction disk space check
   - Checks workdir has 3x archive size before extraction
   - Prevents partial extraction failures when disk fills mid-way
   - Clear error message with required vs available GB

3. --parallel-dbs flag for concurrent restores
   - dbbackup restore cluster archive.tar.gz --parallel-dbs=4
   - Overrides CLUSTER_PARALLELISM config setting
   - Set to 1 for sequential restore (safest for large objects)

cmd/restore.go

@@ -28,6 +28,7 @@ var (
 	restoreClean        bool
 	restoreCreate       bool
 	restoreJobs         int
+	restoreParallelDBs  int // Number of parallel database restores
 	restoreTarget       string
 	restoreVerbose      bool
 	restoreNoProgress   bool
@@ -289,6 +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().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")
@@ -783,6 +785,12 @@ func runRestoreCluster(cmd *cobra.Command, args []string) error {
 		}
 	}
 
+	// Override cluster parallelism if --parallel-dbs is specified
+	if restoreParallelDBs > 0 {
+		cfg.ClusterParallelism = restoreParallelDBs
+		log.Info("Using custom parallelism for database restores", "parallel_dbs", restoreParallelDBs)
+	}
+
 	// Create restore engine
 	engine := restore.New(cfg, log, db)