feat: consume transactional email notifications

Add contract validation, SMTP delivery results, terminal failure context, neutral development seeding, and local Docker setup.

Ref: IT-1033

HrynCo.NotificationService.Worker/SendEmailDeliveryValidator.cs

@@ -0,0 +1,30 @@
+namespace HrynCo.NotificationService.Worker;
+
+using HrynCo.NotificationService.Contracts.Messages;
+using HrynCo.NotificationService.Worker.Services.EmailProcessing;
+using Hrynco.RabbitMq;
+
+internal static class SendEmailDeliveryValidator
+{
+    public static string? GetValidationError(
+        SendEmailMessage message,
+        RabbitMqMessageContext context,
+        string supportedMessageType)
+    {
+        if (string.IsNullOrWhiteSpace(context.MessageId))
+            return "AMQP MessageId is required.";
+
+        if (!string.Equals(context.MessageType, supportedMessageType, StringComparison.Ordinal))
+            return $"Unsupported AMQP message type '{context.MessageType ?? "<missing>"}'.";
+
+        try
+        {
+            SendEmailMessageValidator.Validate(message);
+            return null;
+        }
+        catch (InvalidDataException exception)
+        {
+            return exception.Message;
+        }
+    }
+}