Adds transactional outbox and email verification

Implements outbox pattern for reliable message delivery
Adds email verification flow with Postmark integration
Enhances account registration with secure token generation

Introduces background processing for asynchronous email sending
Implements database-level notification mechanism for message processing

Core/Outbox/IOutboxService.cs

@@ -0,0 +1,13 @@
+#nullable enable
+
+using System.Data;
+
+namespace PlanTempus.Core.Outbox;
+
+public interface IOutboxService
+{
+	Task EnqueueAsync(string type, object payload, IDbConnection? connection = null, IDbTransaction? transaction = null);
+	Task<List<OutboxMessage>> GetPendingAsync(int batchSize = 10);
+	Task MarkAsSentAsync(Guid id);
+	Task MarkAsFailedAsync(Guid id, string errorMessage);
+}