feat: import existing smb.conf and /etc/exports on first boot

Adds auto-detection of pre-existing Samba shares and NFS exports when
nasctl is installed on a host that already has these configs.

New package internal/importer parses smb.conf (INI-style) and
/etc/exports (line-based) and imports them into SQLite.

Imported shares/exports are marked dirty so the user must review
and apply manually before any file is overwritten.

Backup: before the first Apply, each module backs up the original
config to <path>.nasctl.bak.<timestamp> (one time only).

New CLI flag --import-on-boot / NASCTL_IMPORT_ON_BOOT env var
(default false, opt-in).

New API endpoints:
  GET  /api/import/status
  POST /api/import/samba
  POST /api/import/nfs

New DB methods ReplaceSambaShares/ReplaceNFSExports (transactional
replace-all), guarded by import.samba.done / import.nfs.done
settings flags.

internal/modules/nfs/nfs.go

@@ -9,6 +9,7 @@ import (
 	"path/filepath"
 	"strings"
 	"text/template"
+	"time"
 
 	"github.com/darroyo/nasctl/internal/db"
 	"github.com/darroyo/nasctl/internal/system"
@@ -53,6 +54,10 @@ func (m *Module) IsDirty(ctx context.Context, database *db.DB) (bool, error) {
 }
 
 func (m *Module) Apply(ctx context.Context, database *db.DB) error {
+	if err := m.maybeBackup(database); err != nil {
+		return err
+	}
+
 	exports, err := database.ListNFSExports()
 	if err != nil {
 		return err
@@ -131,6 +136,31 @@ func (m *Module) renderConfig(exports []db.NFSExport) ([]byte, error) {
 	return buf.Bytes(), nil
 }
 
+func (m *Module) maybeBackup(database *db.DB) error {
+	done, ok, err := database.GetSetting("backup.nfs.done")
+	if err != nil {
+		return err
+	}
+	if ok && done == "true" {
+		return nil
+	}
+	if _, err := os.Stat(m.cfg.ExportsPath); os.IsNotExist(err) {
+		return nil
+	} else if err != nil {
+		return nil
+	}
+	src, err := os.ReadFile(m.cfg.ExportsPath)
+	if err != nil {
+		return nil
+	}
+	backupPath := m.cfg.ExportsPath + ".nasctl.bak." + time.Now().Format("20060102T150405")
+	if err := os.WriteFile(backupPath, src, 0o644); err != nil {
+		return fmt.Errorf("backup exports: %w", err)
+	}
+	_ = database.SetSetting("backup.nfs.done", "true")
+	return nil
+}
+
 func writeAtomic(path string, content []byte) error {
 	dir := filepath.Dir(path)
 	if err := os.MkdirAll(dir, 0o755); err != nil {