fixed stale state after logout and replayed or lost share intents

lib/api/marianumconnect/queries/timetable_custom_events/custom_events_migration.dart

@@ -31,18 +31,23 @@ class CustomEventsMigration {
     if (DemoMode.active) return;
     if (await _isDone()) return;
 
+    final epoch = AccountData().sessionEpoch;
     try {
       final response = await GetCustomTimetableEvent(
         GetCustomTimetableEventParams(AccountData().getUserSecret()),
       ).run();
 
       for (final event in response.events) {
+        // The POST authenticates with whoever is signed in now; after a
+        // sign-out the previous account's events would land in the next one.
+        if (!AccountData().isCurrentSession(epoch)) return;
         await TimetableCustomEventsAdd().run(event);
         await RemoveCustomTimetableEvent(
           RemoveCustomTimetableEventParams(event.id),
         ).run();
       }
 
+      if (!AccountData().isCurrentSession(epoch)) return;
       await _markDone();
       log('Custom events migration: moved ${response.events.length} event(s) to Marianum-Connect.');
     } catch (e) {