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.Services/PushChannels/SavePushChannelCommand.cs

@@ -0,0 +1,14 @@
+using HrynCo.NotificationService.Services.Core;
+using MediatR;
+
+namespace HrynCo.NotificationService.Services.PushChannels;
+
+// A class deliberately avoids record-generated ToString exposing the submitted secret.
+public sealed class SavePushChannelCommand : IRequest<ServiceResult<Guid>>
+{
+    public Guid Id { get; init; }
+    public Guid ServiceId { get; init; }
+    public required string FirebaseProjectId { get; init; }
+    public string? CredentialsJson { get; init; }
+    public bool IsActive { get; init; }
+}