Fix fire-and-forget TriggerSyncAsync: log exceptions instead of silently discarding them

[davidortinau]

Problem

Every repository in src/SentenceStudio.Shared/Data/ called _syncService?.TriggerSyncAsync().ConfigureAwait(false) as fire-and-forget after SaveChangesAsync(). In .NET 5+, unobserved task exceptions are silently discarded — no crash, no log. .ConfigureAwait(false) on a non-awaited Task is also a no-op. If TriggerSyncAsync threw (connection refused, auth failure, serialization error), the error was lost, the write appeared successful to the caller, and data never synced — with zero indication anything went wrong.

Fix

Added a private void TriggerSync() helper to each affected repository that runs the sync in Task.Run and catches/logs any exception via ILogger:

private void TriggerSync()
{
    if (_syncService is null) return;
    _ = Task.Run(async () =>
    {
        try { await _syncService.TriggerSyncAsync(); }
        catch (Exception ex) { _logger.LogError(ex, "Background sync trigger failed"); }
    });
}

Replaced all 29 silent call sites with TriggerSync().

Changes

File	Sites replaced
LearningResourceRepository.cs	12
ScenarioRepository.cs	2
SkillProfileRepository.cs	2
StoryRepository.cs	2
StreamHistoryRepository.cs	2
UserActivityRepository.cs	2
UserProfileRepository.cs	3
VocabularyLearningContextRepository.cs	2
VocabularyProgressRepository.cs	2
Total	29

Verification

• ✅ SentenceStudio.Shared builds clean (0 errors)
• ✅ 452/452 unit tests pass (dotnet test tests/SentenceStudio.UnitTests)