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/auth.go

@@ -174,11 +174,15 @@ type Server struct {
 	Engine       *engine.Engine
 	AllowedRoots []string
 	Auth         *AuthService
+	SMBConfPath  string
+	ExportsPath  string
 }
 
 type Options struct {
 	AllowedRoots []string
 	Auth         *AuthService
+	SMBConfPath  string
+	ExportsPath  string
 }
 
 func NewServer(database *db.DB, eng *engine.Engine, opts Options) *Server {
@@ -187,6 +191,8 @@ func NewServer(database *db.DB, eng *engine.Engine, opts Options) *Server {
 		Engine:       eng,
 		AllowedRoots: opts.AllowedRoots,
 		Auth:         opts.Auth,
+		SMBConfPath:  opts.SMBConfPath,
+		ExportsPath:  opts.ExportsPath,
 	}
 }