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/AdminPushChannels/Index.cshtml

@@ -0,0 +1,40 @@
+@using HrynCo.NotificationService.DAL.Abstract.PushChannels
+@model IReadOnlyList<PushChannel>
+@{ ViewData["Title"] = "Push Channels"; }
+
+<div class="page-header">
+    <h2><i class="bi bi-phone"></i> Push Channels</h2>
+    <a href="/admin/push-channels/create" class="btn btn-primary btn-sm">Create New Channel</a>
+</div>
+
+@if (Model.Count == 0)
+{
+    <div class="card shadow-sm table-card">
+        <div class="empty-state">
+            <i class="bi bi-phone"></i>
+            <p class="mt-2 mb-0">No push channels found.</p>
+            <a href="/admin/push-channels/create" class="btn btn-primary btn-sm mt-3">Create First Channel</a>
+        </div>
+    </div>
+}
+else
+{
+    <div class="card shadow-sm table-card">
+        <div class="table-responsive">
+            <table class="table table-hover admin-data-table table-sm mb-0">
+                <thead><tr><th scope="col">Service</th><th scope="col">Firebase Project ID</th><th scope="col">Status</th><th scope="col">Actions</th></tr></thead>
+                <tbody>
+                @foreach (var channel in Model)
+                {
+                    <tr>
+                        <td>@channel.ServiceName</td>
+                        <td>@channel.FirebaseProjectId</td>
+                        <td><span class="badge @(channel.IsActive ? "bg-success" : "bg-secondary")">@(channel.IsActive ? "Active" : "Inactive")</span></td>
+                        <td><a href="/admin/push-channels/@channel.Id" class="btn btn-sm btn-link" aria-label="Edit push channel for @channel.ServiceName">Edit</a></td>
+                    </tr>
+                }
+                </tbody>
+            </table>
+        </div>
+    </div>
+}