fix(nfs): assign non-zero fsid during MigrateNFSClients migration

When migrating old clients format (string array) to new format
(NFSClient objects), also fix any exports that have fsid=0 by
generating a new random fsid. This resolves exportfs errors like:
  exportfs: /path fsid=0 requires fsid= for NFS export

The migration now checks if r.FSID == 0 and generates a new one
using system.GenerateRandomFSID() before updating the row.

internal/db/db.go

@@ -9,6 +9,7 @@ import (
 	"sort"
 	"strings"
 
+	"github.com/darroyo/nasctl/internal/system"
 	_ "modernc.org/sqlite"
 )
 
@@ -212,7 +213,15 @@ func (d *DB) MigrateNFSClients() error {
 	}
 
 	for _, r := range toUpdate {
-		if _, err := d.conn.Exec(`UPDATE nfs_exports SET clients = ? WHERE id = ?`, r.Clients, r.ID); err != nil {
+		fsid := r.FSID
+		if fsid == 0 {
+			n, err := system.GenerateRandomFSID()
+			if err != nil {
+				return fmt.Errorf("generate fsid for id %d: %w", r.ID, err)
+			}
+			fsid = int64(n)
+		}
+		if _, err := d.conn.Exec(`UPDATE nfs_exports SET clients = ?, fsid = ? WHERE id = ?`, r.Clients, fsid, r.ID); err != nil {
 			return fmt.Errorf("update nfs export id %d: %w", r.ID, err)
 		}
 	}