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/NotificationDeliveryErrorFormatter.cs

@@ -0,0 +1,80 @@
+namespace HrynCo.NotificationService.Worker.Services.EmailProcessing;
+
+using System.Net.Mail;
+using System.Net.Sockets;
+
+public static class NotificationDeliveryErrorFormatter
+{
+    private const int MaximumErrorLength = 2000;
+    private const string FallbackError = "Notification delivery failed after all retry attempts.";
+
+    public static string Format(Exception exception)
+    {
+        string technicalMessage = Normalize(exception.GetBaseException().Message);
+        if (string.IsNullOrWhiteSpace(technicalMessage))
+        {
+            return FallbackError;
+        }
+
+        string contextualMessage = CreateContextualMessage(exception, technicalMessage);
+        return contextualMessage.Length <= MaximumErrorLength
+            ? contextualMessage
+            : contextualMessage[..MaximumErrorLength];
+    }
+
+    private static string CreateContextualMessage(Exception exception, string technicalMessage)
+    {
+        if (FindException<SmtpFailedRecipientException>(exception) is not null)
+        {
+            return "SMTP delivery failed after all retry attempts: " +
+                   "the SMTP server rejected the recipient address.";
+        }
+
+        SocketException? socketException = FindException<SocketException>(exception);
+        if (socketException is not null)
+        {
+            string reason = socketException.SocketErrorCode switch
+            {
+                SocketError.HostNotFound or SocketError.NoData =>
+                    "the configured SMTP server host could not be resolved",
+                SocketError.ConnectionRefused =>
+                    "the configured SMTP server refused the connection",
+                SocketError.TimedOut =>
+                    "the connection to the configured SMTP server timed out",
+                _ when technicalMessage.Contains(
+                    "Name or service not known",
+                    StringComparison.OrdinalIgnoreCase) =>
+                    "the configured SMTP server host could not be resolved",
+                _ => "the configured SMTP server could not be reached"
+            };
+
+            return $"SMTP delivery failed after all retry attempts: {reason} ({technicalMessage}).";
+        }
+
+        if (FindException<SmtpException>(exception) is not null)
+        {
+            return $"SMTP delivery failed after all retry attempts ({technicalMessage}).";
+        }
+
+        return $"Notification delivery failed after all retry attempts ({technicalMessage}).";
+    }
+
+    private static TException? FindException<TException>(Exception exception)
+        where TException : Exception
+    {
+        for (Exception? current = exception; current is not null; current = current.InnerException)
+        {
+            if (current is TException typedException)
+            {
+                return typedException;
+            }
+        }
+
+        return null;
+    }
+
+    private static string Normalize(string message)
+    {
+        return message.ReplaceLineEndings(" ").Trim();
+    }
+}