fix: populate fsid for legacy NFS exports on migration

Migration 0002 added fsid column with DEFAULT 0 but never
populated existing rows. Now PopulateLegacyFSIDs() runs after 0002
to extract fsid from legacy options string or generate random.

Also: move generateRandomFSID to internal/system to avoid import
cycles. Migration 0004 drops the legacy options column.

Version: 0.3.1 -> 0.3.2

internal/modules/nfs/fsid_test.go

@@ -3,6 +3,8 @@ package nfs
 import (
 	"sync"
 	"testing"
+
+	"github.com/darroyo/nasctl/internal/system"
 )
 
 func TestUniqueFSIDNoCollision(t *testing.T) {
@@ -14,7 +16,7 @@ func TestUniqueFSIDNoCollision(t *testing.T) {
 		wg.Add(1)
 		go func() {
 			defer wg.Done()
-			n, err := generateRandomFSID()
+			n, err := system.GenerateRandomFSID()
 			if err != nil {
 				t.Errorf("generateRandomFSID failed: %v", err)
 				return