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/EmailTemplateRenderingService.cs

@@ -8,6 +8,18 @@ internal sealed class EmailTemplateRenderingService : IEmailTemplateRenderingSer
 {
     public RenderedEmail Render(EmailTemplate template, SendEmailMessageData data)
     {
+        string[] missingVariables = template.Variables
+            .Where(variable => variable.Required)
+            .Select(variable => variable.Name)
+            .Where(name => !data.Variables.TryGetValue(name, out string? value) || string.IsNullOrWhiteSpace(value))
+            .ToArray();
+
+        if (missingVariables.Length > 0)
+        {
+            throw new InvalidDataException(
+                $"Required template variables are missing: {string.Join(", ", missingVariables)}.");
+        }
+
         return new RenderedEmail(
             Interpolate(template.Subject, data.Variables),
             Interpolate(template.HtmlBody, data.Variables),