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/OutboxMessage.cs

@@ -0,0 +1,27 @@
+#nullable enable
+
+namespace PlanTempus.Core.Outbox;
+
+public class OutboxMessage
+{
+	public Guid Id { get; set; }
+	public required string Type { get; set; }
+	public required object Payload { get; set; }
+	public string Status { get; set; } = "pending";
+	public DateTime CreatedAt { get; set; }
+	public DateTime? ProcessedAt { get; set; }
+	public int RetryCount { get; set; }
+	public string? ErrorMessage { get; set; }
+}
+
+public static class OutboxMessageTypes
+{
+	public const string VerificationEmail = "verification_email";
+}
+
+public class VerificationEmailPayload
+{
+	public required string Email { get; set; }
+	public required string UserName { get; set; }
+	public required string Token { get; set; }
+}