feat: add export and backup commands

- nib export: dump all entities to JSON (stdout or --output file)
- nib backup: atomic SQLite backup via VACUUM INTO (WAL-safe)
- Store.Backup() method on db layer
- Tests for both commands

cmd/backup.go

@@ -0,0 +1,46 @@
+package cmd
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/lerko/nib/internal/db"
+	"github.com/spf13/cobra"
+)
+
+var backupCmd = &cobra.Command{
+	Use:   "backup [path]",
+	Short: "create a safe backup of the database",
+	Long:  "Creates an atomic backup using VACUUM INTO. Safe with WAL mode — no need to stop the server.",
+	Args:  cobra.MaximumNArgs(1),
+	RunE:  runBackup,
+}
+
+func init() {
+	rootCmd.AddCommand(backupCmd)
+}
+
+func runBackup(cmd *cobra.Command, args []string) error {
+	srcPath, err := db.DefaultPath()
+	if err != nil {
+		return err
+	}
+
+	dst := fmt.Sprintf("%s.backup-%s", srcPath, time.Now().Format("20060102-150405"))
+	if len(args) > 0 {
+		dst = args[0]
+	}
+
+	store, err := db.Open(srcPath)
+	if err != nil {
+		return err
+	}
+	defer store.Close()
+
+	if err := store.Backup(dst); err != nil {
+		return fmt.Errorf("backup failed: %w", err)
+	}
+
+	fmt.Printf("backed up to %s\n", dst)
+	return nil
+}