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.

AGENTS.md

@@ -43,6 +43,7 @@ No separate frontend test command; no lint/typecheck Makefile targets.
 | `NASCTL_EXPORTS` | `./.data/exports` | `/etc/exports` |
 | `NASCTL_EXEC_SYSTEM` | `false` | `true` |
 | `NASCTL_ALLOWED_ROOTS` | (empty) | (empty = any absolute path) |
+| `NASCTL_IMPORT_ON_BOOT` | `false` | `true` to import existing `smb.conf` and `/etc/exports` on first boot |
 
 ## Frontend Dev