feat(push): add service-owned firebase channels and delivery

Add push contracts, provider delivery and channel diagnostics. Link email channels to services and document migration and validation.

Ref: IT-1074

HrynCo.NotificationService.Worker/SendPushConsumer.cs

@@ -0,0 +1,37 @@
+namespace HrynCo.NotificationService.Worker;
+
+using HrynCo.NotificationService.Contracts.Messages;
+using HrynCo.NotificationService.Worker.Services.Push;
+using Hrynco.RabbitMq;
+using Microsoft.Extensions.Options;
+
+public sealed class SendPushConsumer(
+    IOptionsMonitor<RabbitMqSettings> options, IServiceScopeFactory scopes,
+    ILogger<SendPushConsumer> logger)
+    : RabbitMqConsumerBase<SendPushMessage, SendPushMessageData>(options, logger)
+{
+    protected override string QueueName => "notification.send-push";
+
+    protected override bool TryValidateMessage(SendPushMessage message, RabbitMqMessageContext context, out string? validationError)
+    {
+        bool valid = SendPushDeliveryValidator.IsValid(message, context);
+        validationError = valid ? null : "Invalid push envelope.";
+        return valid;
+    }
+
+    protected override async Task HandleMessageAsync(SendPushMessage message, RabbitMqMessageContext context,
+        CancellationToken cancellationToken)
+    {
+        await using var scope = scopes.CreateAsyncScope();
+        await scope.ServiceProvider.GetRequiredService<PushDeliveryService>()
+            .ProcessAsync(Guid.Parse(context.MessageId!), message, cancellationToken);
+    }
+
+    protected override async Task HandleMessageRetriesExhaustedAsync(SendPushMessage message,
+        RabbitMqMessageContext context, Exception exception, CancellationToken cancellationToken)
+    {
+        await using var scope = scopes.CreateAsyncScope();
+        await scope.ServiceProvider.GetRequiredService<PushDeliveryService>()
+            .CompleteFailureAsync(Guid.Parse(context.MessageId!), message, cancellationToken);
+    }
+}