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/importer/nfs.go

@@ -3,8 +3,6 @@ package importer
 import (
 	"bufio"
 	"bytes"
-	"crypto/rand"
-	"encoding/binary"
 	"encoding/json"
 	"fmt"
 	"os"
@@ -170,14 +168,10 @@ func parseExportLine(line string) (db.NFSExport, bool) {
 }
 
 func generateImportFSID() (int64, error) {
-	var b [4]byte
-	if _, err := rand.Read(b[:]); err != nil {
+	n, err := system.GenerateRandomFSID()
+	if err != nil {
 		return 0, err
 	}
-	n := binary.BigEndian.Uint32(b[:])
-	if n == 0 {
-		n = 1
-	}
 	return int64(n), nil
 }