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/system/fsid.go

@@ -0,0 +1,19 @@
+package system
+
+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
+}