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.Services/EmailProcessing/SendEmailMessageValidator.cs

@@ -0,0 +1,36 @@
+namespace HrynCo.NotificationService.Worker.Services.EmailProcessing;
+
+using System.Net.Mail;
+using HrynCo.NotificationService.Contracts.Messages;
+
+public static class SendEmailMessageValidator
+{
+    public static void Validate(SendEmailMessage message)
+    {
+        ArgumentNullException.ThrowIfNull(message);
+
+        if (message.CorrelationContext is null)
+            throw new InvalidDataException("CorrelationContext is required.");
+        if (string.IsNullOrWhiteSpace(message.CorrelationContext.CorrelationId))
+            throw new InvalidDataException("CorrelationContext.CorrelationId is required.");
+        if (message.Data is null)
+            throw new InvalidDataException("Data is required.");
+
+        Require(message.Data.ServiceName, nameof(message.Data.ServiceName));
+        Require(message.Data.TemplateKey, nameof(message.Data.TemplateKey));
+        Require(message.Data.RecipientEmail, nameof(message.Data.RecipientEmail));
+        Require(message.Data.RecipientName, nameof(message.Data.RecipientName));
+        Require(message.Data.LanguageCode, nameof(message.Data.LanguageCode));
+
+        if (!MailAddress.TryCreate(message.Data.RecipientEmail, out _))
+            throw new InvalidDataException("RecipientEmail is not a valid email address.");
+        if (message.Data.Variables is null)
+            throw new InvalidDataException("Variables is required.");
+    }
+
+    private static void Require(string? value, string fieldName)
+    {
+        if (string.IsNullOrWhiteSpace(value))
+            throw new InvalidDataException($"{fieldName} is required.");
+    }
+}