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/cmd_test.go

@@ -3,6 +3,8 @@ package cmd
 import (
 	"bytes"
 	"context"
+	"encoding/json"
+	"os"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -219,3 +221,66 @@ func TestRunLsEmpty(t *testing.T) {
 		t.Fatalf("runLs empty: %v", err)
 	}
 }
+
+func TestRunExport(t *testing.T) {
+	store := testStore(t)
+	seedEntity(t, store, "export me", db.GlyphNote)
+	seedEntity(t, store, "export me too", db.GlyphTodo)
+	store.Close()
+
+	outFile := filepath.Join(t.TempDir(), "export.json")
+	exportOutput = outFile
+
+	err := runExport(newCmd(), nil)
+	if err != nil {
+		t.Fatalf("runExport: %v", err)
+	}
+
+	data, err := os.ReadFile(outFile)
+	if err != nil {
+		t.Fatalf("read export: %v", err)
+	}
+
+	var entities []exportEntity
+	if err := json.Unmarshal(data, &entities); err != nil {
+		t.Fatalf("unmarshal export: %v", err)
+	}
+
+	if len(entities) != 2 {
+		t.Fatalf("expected 2 entities, got %d", len(entities))
+	}
+}
+
+func TestRunBackup(t *testing.T) {
+	store := testStore(t)
+	seedEntity(t, store, "backup me", db.GlyphNote)
+	store.Close()
+
+	dst := filepath.Join(t.TempDir(), "backup.db")
+	err := runBackup(newCmd(), []string{dst})
+	if err != nil {
+		t.Fatalf("runBackup: %v", err)
+	}
+
+	info, err := os.Stat(dst)
+	if err != nil {
+		t.Fatalf("backup file missing: %v", err)
+	}
+	if info.Size() == 0 {
+		t.Fatal("backup file is empty")
+	}
+
+	backed, err := db.Open(dst)
+	if err != nil {
+		t.Fatalf("open backup: %v", err)
+	}
+	defer backed.Close()
+
+	entities, err := backed.List(context.Background(), db.DefaultListParams())
+	if err != nil {
+		t.Fatalf("list backup: %v", err)
+	}
+	if len(entities) != 1 {
+		t.Fatalf("expected 1 entity in backup, got %d", len(entities))
+	}
+}