feat(application): add IOAuth2TokenService and IPop3EmailService interfaces

src/core/DigitalData.MessagingService.Application/Common/Interfaces/IOAuth2TokenService.cs

@@ -0,0 +1,18 @@
+#if NET
+using DigitalData.MessagingService.Domain.Entities;
+
+namespace DigitalData.MessagingService.Application.Common.Interfaces;
+
+/// <summary>
+/// Acquires and caches OAuth2 access tokens for email protocols (IMAP, POP3, SMTP).
+/// Uses the client credentials flow (application-level auth — no user interaction required).
+/// </summary>
+public interface IOAuth2TokenService
+{
+    /// <summary>
+    /// Returns a valid access token for the given email account.
+    /// Tokens are cached and refreshed automatically before expiry.
+    /// </summary>
+    Task<string> GetAccessTokenAsync(EmailAccount account, CancellationToken cancellationToken = default);
+}
+#endif

src/core/DigitalData.MessagingService.Application/Common/Interfaces/IPop3EmailService.cs

@@ -0,0 +1,25 @@
+#if NET
+using DigitalData.MessagingService.Application.Common.Dto;
+using DigitalData.MessagingService.Domain.Entities;
+
+namespace DigitalData.MessagingService.Application.Common.Interfaces;
+
+/// <summary>
+/// Service interface for reading emails via POP3.
+/// </summary>
+public interface IPop3EmailService
+{
+    /// <summary>
+    /// Fetches new messages from the POP3 server and persists them locally.
+    /// Because POP3 has no folder concept, all messages are stored under the folder name "INBOX".
+    /// </summary>
+    Task<EmailSyncResult> SyncEmailsAsync(
+        EmailAccount account,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Gets the last POP3 sync date for the specified account.
+    /// </summary>
+    DateTime? GetLastPop3SyncDate(int accountId);
+}
+#endif