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/Views/AdminServices/DeleteService.cshtml

@@ -14,7 +14,7 @@ else
 {
     <div class="alert alert-danger">This permanently deletes the service and its language catalog.</div>
 }
-<p>Email channels are retained. Their service names are independent and are not renamed or deleted.</p>
+<p>Reassign this service's push channel and reassign or remove its email channels before deleting the service. Channels are never deleted automatically.</p>
 <form asp-action="DeleteService" asp-route-serviceId="@Model.ServiceId" method="post">
     @Html.AntiForgeryToken()
     <input asp-for="Revision" type="hidden" />

HrynCo.NotificationService.Web/Views/AdminServices/Edit.cshtml

@@ -15,7 +15,7 @@
     <label asp-for="Name" class="form-label fw-semibold">Service name</label>
     <input asp-for="Name" class="form-control mb-3" maxlength="100" />
     <h3 class="h5">Language codes</h3>
-    <p class="text-muted">Codes are case-sensitive and belong to this service. Existing template versions use these codes when sending. Renaming a service or code changes sender lookup; align publishers and email channel service names.</p>
+    <p class="text-muted">Codes are case-sensitive and belong to this service. Existing template versions use these codes when sending. Renaming a service or code changes sender lookup; update publishers to use the current names. Email channels remain linked to this service automatically.</p>
     <div class="table-responsive">
         <table class="table admin-data-table service-language-table"><thead><tr><th scope="col">Code</th><th scope="col">Versions</th><th scope="col" class="service-language-actions">Actions</th></tr></thead><tbody id="serviceLanguages">
         @for (var i = 0; i < Model.Languages.Count; i++)