Refactored all synchronizers to deal with some of the chronic issues.

Wino.Core/Synchronizers/Errors/Gmail/GmailHistoryExpiredHandler.cs

@@ -0,0 +1,78 @@
+using System;
+using System.Threading.Tasks;
+using Google;
+using Serilog;
+using Wino.Core.Domain.Enums;
+using Wino.Core.Domain.Interfaces;
+using Wino.Core.Domain.Models.Synchronization;
+using Wino.Core.Integration.Processors;
+
+namespace Wino.Core.Synchronizers.Errors.Gmail;
+
+/// <summary>
+/// Handles Gmail history ID expiration errors.
+/// When history is no longer available, resets the account's history ID to force a full resync.
+/// </summary>
+public class GmailHistoryExpiredHandler : ISynchronizerErrorHandler
+{
+    private readonly ILogger _logger = Log.ForContext<GmailHistoryExpiredHandler>();
+    private readonly IGmailChangeProcessor _gmailChangeProcessor;
+
+    public GmailHistoryExpiredHandler(IGmailChangeProcessor gmailChangeProcessor)
+    {
+        _gmailChangeProcessor = gmailChangeProcessor;
+    }
+
+    public bool CanHandle(SynchronizerErrorContext error)
+    {
+        // Gmail returns 404 when history ID is no longer valid
+        if (error.ErrorCode == 404)
+        {
+            var message = error.ErrorMessage?.ToLowerInvariant() ?? string.Empty;
+            return message.Contains("history") || message.Contains("notfound");
+        }
+
+        if (error.Exception is GoogleApiException googleEx)
+        {
+            if (googleEx.HttpStatusCode == System.Net.HttpStatusCode.NotFound)
+            {
+                var errorMessage = googleEx.Message?.ToLowerInvariant() ?? string.Empty;
+                return errorMessage.Contains("history") ||
+                       errorMessage.Contains("not found") ||
+                       errorMessage.Contains("starthistoryid");
+            }
+        }
+
+        return false;
+    }
+
+    public async Task<bool> HandleAsync(SynchronizerErrorContext error)
+    {
+        _logger.Warning(error.Exception,
+            "Gmail history ID expired for account {AccountName} ({AccountId}). Resetting to force full sync.",
+            error.Account?.Name, error.Account?.Id);
+
+        error.Severity = SynchronizerErrorSeverity.Recoverable;
+        error.Category = SynchronizerErrorCategory.ResourceNotFound;
+
+        // Reset the account's synchronization identifier (history ID)
+        if (error.Account != null)
+        {
+            try
+            {
+                await _gmailChangeProcessor.UpdateAccountDeltaSynchronizationIdentifierAsync(
+                    error.Account.Id, string.Empty).ConfigureAwait(false);
+
+                _logger.Information("Successfully reset Gmail history ID for account {AccountName}. Next sync will be full sync.",
+                    error.Account.Name);
+            }
+            catch (Exception ex)
+            {
+                _logger.Error(ex, "Failed to reset Gmail history ID for account {AccountName}",
+                    error.Account.Name);
+            }
+        }
+
+        return true;
+    }
+}

Wino.Core/Synchronizers/Errors/Gmail/GmailQuotaExceededHandler.cs

@@ -0,0 +1,57 @@
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Google;
+using Serilog;
+using Wino.Core.Domain.Enums;
+using Wino.Core.Domain.Interfaces;
+using Wino.Core.Domain.Models.Synchronization;
+
+namespace Wino.Core.Synchronizers.Errors.Gmail;
+
+/// <summary>
+/// Handles Gmail API quota exceeded errors (HTTP 403 with quota error).
+/// This is a more severe rate limit that indicates daily quota exhaustion.
+/// </summary>
+public class GmailQuotaExceededHandler : ISynchronizerErrorHandler
+{
+    private readonly ILogger _logger = Log.ForContext<GmailQuotaExceededHandler>();
+
+    public bool CanHandle(SynchronizerErrorContext error)
+    {
+        if (error.Exception is GoogleApiException googleEx)
+        {
+            // Quota exceeded usually returns 403
+            if (googleEx.HttpStatusCode == System.Net.HttpStatusCode.Forbidden)
+            {
+                var errorMessage = googleEx.Message?.ToLowerInvariant() ?? string.Empty;
+                var errorReason = googleEx.Error?.Errors?.FirstOrDefault()?.Reason?.ToLowerInvariant() ?? string.Empty;
+
+                return errorMessage.Contains("quota") ||
+                       errorMessage.Contains("limit exceeded") ||
+                       errorReason.Contains("quota") ||
+                       errorReason.Contains("ratelimitexceeded") ||
+                       errorReason.Contains("userlimitexceeded");
+            }
+        }
+
+        return false;
+    }
+
+    public Task<bool> HandleAsync(SynchronizerErrorContext error)
+    {
+        _logger.Warning(error.Exception,
+            "Gmail API quota exceeded for account {AccountName} ({AccountId}). Sync will be paused.",
+            error.Account?.Name, error.Account?.Id);
+
+        // Quota exceeded is more severe - treat as fatal to prevent repeated failures
+        // The user will be notified and sync will resume after quota resets
+        error.Severity = SynchronizerErrorSeverity.Fatal;
+        error.Category = SynchronizerErrorCategory.RateLimit;
+
+        // Suggest a very long delay - quotas typically reset daily
+        error.RetryDelay = TimeSpan.FromHours(1);
+
+        return Task.FromResult(true);
+    }
+}

Wino.Core/Synchronizers/Errors/Gmail/GmailRateLimitHandler.cs

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+using System;
+using System.Threading.Tasks;
+using Google;
+using Serilog;
+using Wino.Core.Domain.Enums;
+using Wino.Core.Domain.Interfaces;
+using Wino.Core.Domain.Models.Synchronization;
+
+namespace Wino.Core.Synchronizers.Errors.Gmail;
+
+/// <summary>
+/// Handles Gmail API rate limiting errors (HTTP 429 Too Many Requests).
+/// Marks the error as transient with appropriate backoff delay.
+/// </summary>
+public class GmailRateLimitHandler : ISynchronizerErrorHandler
+{
+    private readonly ILogger _logger = Log.ForContext<GmailRateLimitHandler>();
+
+    public bool CanHandle(SynchronizerErrorContext error)
+    {
+        if (error.ErrorCode == 429)
+            return true;
+
+        if (error.Exception is GoogleApiException googleEx)
+        {
+            return googleEx.HttpStatusCode == System.Net.HttpStatusCode.TooManyRequests ||
+                   (googleEx.Error?.Code == 429);
+        }
+
+        return false;
+    }
+
+    public Task<bool> HandleAsync(SynchronizerErrorContext error)
+    {
+        _logger.Warning(error.Exception,
+            "Gmail API rate limit hit for account {AccountName} ({AccountId}). Operation: {Operation}. Will retry with backoff.",
+            error.Account?.Name, error.Account?.Id, error.OperationType ?? "N/A");
+
+        error.Severity = SynchronizerErrorSeverity.Transient;
+        error.Category = SynchronizerErrorCategory.RateLimit;
+
+        // Gmail rate limits are usually per-user, suggest a longer delay
+        error.RetryDelay = TimeSpan.FromSeconds(10);
+
+        return Task.FromResult(true);
+    }
+}