Add EmailSyncWorker and enhance IMAP service with caching

Refactored `ReceivedEmailContext` to use `record` for immutability
and value-based equality. Added `SyncIntervalSeconds` to
`EmailAccountsOptions` for configuring IMAP sync intervals.

Introduced `EmailSyncWorker` as a background service for periodic
email synchronization. Registered `IMemoryCache` and integrated
caching in `LimilabsImapEmailService` to reduce redundant fetch
operations. Optimized `FetchEmailByUidAsync` to conditionally
handle attachments and improve performance.

Refactored logging and improved code readability by adopting modern
C# features like `record`, `with` expressions, and `IOptions`.
Performed general cleanup and streamlined method implementations.

src/infrastructure/DigitalData.MessagingService.Infrastructure/DependencyInjection.cs

@@ -50,6 +50,9 @@ public static class DependencyInjection
 
         // Register Background Workers
         services.AddHostedService<AsyncInitWorker>();
+        services.AddHostedService<EmailSyncWorker>();
+
+        services.AddMemoryCache();
 
         return services;
     }