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.Web.IntegrationTests/PushDeliveryRepositoryTests.cs

@@ -0,0 +1,34 @@
+namespace HrynCo.NotificationService.Web.IntegrationTests;
+
+using HrynCo.NotificationService.DAL.Abstract.Push;
+using HrynCo.NotificationService.DAL.EF;
+using Microsoft.Extensions.DependencyInjection;
+
+public sealed class PushDeliveryRepositoryTests(TemplateDatabaseFixture fixture) : IClassFixture<TemplateDatabaseFixture>
+{
+    [Fact]
+    public async Task RedeliveryReplaysOutcomeAndFailedProviderCallCanRetry()
+    {
+        var connection = await fixture.CreateDatabaseAsync();
+        using var provider = new ServiceCollection().AddNotificationDataAccess(connection).BuildServiceProvider();
+        var id = Guid.NewGuid();
+        using (var first = provider.CreateScope())
+        {
+            var repository = first.ServiceProvider.GetRequiredService<IPushDeliveryRepository>();
+            await Assert.ThrowsAsync<InvalidOperationException>(() => repository.DeliverOnceAsync(id, "Invemory",
+                _ => throw new InvalidOperationException("transient"), default));
+        }
+        using (var retry = provider.CreateScope())
+        {
+            var repository = retry.ServiceProvider.GetRequiredService<IPushDeliveryRepository>();
+            var result = await repository.DeliverOnceAsync(id, "Invemory",
+                _ => Task.FromResult(new PushDeliveryOutcome("unregistered", true)), default);
+            Assert.True(result.InvalidRegistration);
+        }
+        using var redelivery = provider.CreateScope();
+        var saved = await redelivery.ServiceProvider.GetRequiredService<IPushDeliveryRepository>()
+            .DeliverOnceAsync(id, "Invemory", _ => throw new Exception("must not send twice"), default);
+        Assert.Equal("unregistered", saved.Error);
+        Assert.True(saved.InvalidRegistration);
+    }
+}