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/core/DigitalData.MessagingService.Application/Common/Options/EmailAccountsOptions.cs

@@ -14,4 +14,10 @@ public class EmailAccountsOptions
     /// The list of configured email accounts.
     /// </summary>
     public required IEnumerable<EmailAccountDto> Accounts { get; init; } = [];
+
+    /// <summary>
+    /// How often the IMAP sync worker polls for new emails, in seconds.
+    /// Defaults to 300 seconds (5 minutes).
+    /// </summary>
+    public int SyncIntervalSeconds { get; init; } = 300;
 }