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.go

@@ -1,26 +1,14 @@
 package nfs
 
 import (
-	"crypto/rand"
-	"encoding/binary"
 	"fmt"
-)
 
-func generateRandomFSID() (uint32, error) {
-	var b [4]byte
-	if _, err := rand.Read(b[:]); err != nil {
-		return 0, fmt.Errorf("crypto/rand read: %w", err)
-	}
-	n := binary.BigEndian.Uint32(b[:])
-	if n == 0 {
-		n = 1
-	}
-	return n, nil
-}
+	"github.com/darroyo/nasctl/internal/system"
+)
 
 func UniqueFSID(used map[uint32]struct{}) (uint32, error) {
 	for i := 0; i < 64; i++ {
-		n, err := generateRandomFSID()
+		n, err := system.GenerateRandomFSID()
 		if err != nil {
 			return 0, err
 		}