feat(admin-console): Stage 6 — mail & notifications domain

Add the mail, notifications, and broadcast pages over the mail, notification,
and diplomail services (no new business logic), completing the operator console.

- GET  /_gm/mail                         deliveries (paginated) + dead-letters
- GET  /_gm/mail/deliveries/{id}         delivery detail + attempts
- POST /_gm/mail/deliveries/{id}/resend  re-enqueue a non-sent delivery
- GET  /_gm/notifications                notifications + dead-letters + malformed
- GET/POST /_gm/broadcast                multi-game admin diplomatic broadcast

Console depends on MailAdmin / NotificationAdmin / DiplomailAdmin interfaces
(satisfied by the concrete services); pages render in tests without a database.
Delivery detail and dead-letters live under /_gm/mail/deliveries/* and
/_gm/mail/... static segments to avoid a param/static route conflict. Resend
and broadcast flow through the CSRF guard.

Tests: mail page, delivery detail (+ not-found), resend (+ bad-CSRF),
notifications overview, broadcast form + send (input assertions) + bad game
ids, and unavailable. Plus an integration test that drives /_gm end to end
through the real gateway → backend (401 challenge + authenticated dashboard).

Docs: backend/docs/admin-console.md page inventory completed.

backend/internal/adminconsole/templates/pages/notifications.gohtml

@@ -0,0 +1,27 @@
+{{define "content" -}}
+<h1>Notifications</h1>
+<nav class="subnav"><a href="/_gm/mail">Deliveries</a> · <a href="/_gm/notifications">Notifications</a> · <a href="/_gm/broadcast">Broadcast</a></nav>
+{{with .Data}}
+<section class="panel">
+<h2>Recent notifications</h2>
+<table class="list"><thead><tr><th>ID</th><th>Kind</th><th>User</th><th>Created</th></tr></thead><tbody>
+{{range .Notifications}}<tr><td><code>{{.NotificationID}}</code></td><td>{{.Kind}}</td><td>{{.UserID}}</td><td>{{.Created}}</td></tr>
+{{else}}<tr><td colspan="4"><span class="note">none</span></td></tr>{{end}}
+</tbody></table>
+</section>
+<section class="panel">
+<h2>Dead-letters</h2>
+<table class="list"><thead><tr><th>Notification</th><th>Route</th><th>Reason</th><th>Archived</th></tr></thead><tbody>
+{{range .DeadLetters}}<tr><td><code>{{.NotificationID}}</code></td><td><code>{{.RouteID}}</code></td><td>{{.Reason}}</td><td>{{.Archived}}</td></tr>
+{{else}}<tr><td colspan="4"><span class="note">none</span></td></tr>{{end}}
+</tbody></table>
+</section>
+<section class="panel">
+<h2>Malformed intents</h2>
+<table class="list"><thead><tr><th>ID</th><th>Reason</th><th>Received</th></tr></thead><tbody>
+{{range .Malformed}}<tr><td><code>{{.ID}}</code></td><td>{{.Reason}}</td><td>{{.Received}}</td></tr>
+{{else}}<tr><td colspan="3"><span class="note">none</span></td></tr>{{end}}
+</tbody></table>
+</section>
+{{end}}
+{{- end}}