feat(notification): make messaging consumer configurable

Replace the hard-coded RabbitMQ setup with NotificationOptions bound from a configurable section. The new ConsumerType switch wires either the HTTP API client or the RabbitMQ client, and startup now fails with a clear error when required settings are missing.

Bump DigitalData.MessagingService.Client to 3.0.1 for the abstraction used by both transports.

EnvelopeGenerator.Infrastructure.Notification/Configuration/NotificationOptions.cs

@@ -0,0 +1,16 @@
+using DigitalData.MessagingService.RabbitMQ;
+
+namespace EnvelopeGenerator.Infrastructure.Notification.Configuration;
+
+public class NotificationOptions
+{
+    public const string SectionName = "Notification";
+
+    public ConsumerType ConsumerType { get; set; }
+
+    public string? ApiUrl { get; set; }
+
+    public string SenderEmail { get; set; } = null!;
+
+    public RabbitMqConnectionConfiguration? RabbitMQ { get; set; }
+}