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/web/router.go

@@ -30,6 +30,10 @@ func NewRouter(s *Server) chi.Router {
 			protected.Get("/apply/log", s.handleApplyLog)
 			protected.Get("/system/status", s.handleSystemStatus)
 
+			protected.Get("/import/status", s.handleImportStatus)
+			protected.Post("/import/samba", s.handleImportSamba)
+			protected.Post("/import/nfs", s.handleImportNFS)
+
 			protected.Route("/samba/shares", func(shares chi.Router) {
 				shares.Get("/", s.handleListSambaShares)
 				shares.Post("/", s.handleCreateSambaShare)