Refactor IMAP sync caching and improve thread safety

Replaced `IMemoryCache` with a thread-safe `ConcurrentDictionary`
for managing IMAP sync dates in `LimilabsImapEmailService`. Added
`GetLastImapSyncDate` and `SetLastImapSyncDate` methods to handle
cache operations. Updated the `IImapEmailService` interface to
include `GetLastImapSyncDate`. Simplified the `MarkAsSeenAsync`
method signature for better readability. Introduced a private
record type `ImapCacheKey` to encapsulate cache keys.

src/core/DigitalData.MessagingService.Application/Common/Interfaces/IImapEmailService.cs

@@ -29,5 +29,13 @@ public interface IImapEmailService
         long uid,
         string folder = "INBOX",
         CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Gets the last IMAP sync date for the specified account and folder.
+    /// </summary>
+    /// <param name="accountId"></param>
+    /// <param name="folder"></param>
+    /// <returns></returns>
+    DateTime? GetLastImapSyncDate(int accountId, string folder = "INBOX");
 }
 #endif

src/infrastructure/DigitalData.MessagingService.Infrastructure/Services/LimilabsImapEmailService.cs

@@ -5,8 +5,8 @@ using DigitalData.MessagingService.Domain.Entities;
 using DigitalData.MessagingService.Infrastructure.Services.Extensions;
 using Limilabs.Client.IMAP;
 using Limilabs.Mail;
-using Microsoft.Extensions.Caching.Memory;
 using Microsoft.Extensions.Logging;
+using System.Collections.Concurrent;
 using System.Text;
 
 namespace DigitalData.MessagingService.Infrastructure.Services;
@@ -15,7 +15,7 @@ namespace DigitalData.MessagingService.Infrastructure.Services;
 /// IMAP email service using Limilabs Mail.dll.
 /// Opens a fresh connection per call — stateless and thread-safe.
 /// </summary>
-public class LimilabsImapEmailService(ILogger<LimilabsImapEmailService> Logger, IRepository<ReceivedEmail> Repository, IMemoryCache Cache) : IImapEmailService
+public class LimilabsImapEmailService(ILogger<LimilabsImapEmailService> Logger, IRepository<ReceivedEmail> Repository) : IImapEmailService
 {
     private static readonly string CacheKeyPrefix = Guid.NewGuid().ToString();
 
@@ -34,7 +34,7 @@ public class LimilabsImapEmailService(ILogger<LimilabsImapEmailService> Logger,
             // Server-side: only date range; all other filters are applied in-process after cache retrieval
             List<ICriterion> criterions = [];
 
-            var since = Cache.GetLastImapSyncDate(account.Id, folder);
+            var since = GetLastImapSyncDate(account.Id, folder);
 
             if (since is not null && since != default)
                 criterions.Add(Expression.SentSince(since.Value));
@@ -112,7 +112,7 @@ public class LimilabsImapEmailService(ILogger<LimilabsImapEmailService> Logger,
 
                     emails.Add(email);
 
-                    Cache.SetLastImapSyncDate(account.Id, folder, operationStartTime);
+                    SetLastImapSyncDate(account.Id, folder, operationStartTime);
                 }
                 catch (Exception ex)
                 {
@@ -136,11 +136,7 @@ public class LimilabsImapEmailService(ILogger<LimilabsImapEmailService> Logger,
         }
     }
 
-    public async Task MarkAsSeenAsync(
-        EmailAccount account,
-        long uid,
-        string folder = "INBOX",
-        CancellationToken cancel = default)
+    public async Task MarkAsSeenAsync(EmailAccount account, long uid, string folder = "INBOX", CancellationToken cancel = default)
     {
         using var imap = await OpenAsync(account, folder, cancel);
         try
@@ -173,4 +169,21 @@ public class LimilabsImapEmailService(ILogger<LimilabsImapEmailService> Logger,
 
         return imap;
     }
+
+    #region IMAP Last Sync Date Cache
+    private readonly ConcurrentDictionary<ImapCacheKey, DateTime> _cache = new();
+
+    private record ImapCacheKey(int AccountId, string Folder);
+
+    public DateTime? GetLastImapSyncDate(int accountId, string folder = "INBOX")
+    {
+        return _cache.GetValueOrDefault(new ImapCacheKey(accountId, folder));
+    }
+
+    private void SetLastImapSyncDate(int accountId, string folder, DateTime date)
+    {
+        var key = new ImapCacheKey(accountId, folder);
+        _cache[key] = date;
+    }
+    #endregion
 }