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/mail_delivery.gohtml

@@ -0,0 +1,33 @@
+{{define "content" -}}
+{{$csrf := .CSRFToken}}
+{{with .Data}}
+<p><a href="/_gm/mail">&laquo; mail</a></p>
+<h1>Delivery</h1>
+<section class="panel">
+<ul class="kv">
+<li>Delivery ID: <code>{{.DeliveryID}}</code></li>
+<li>Template: {{.Template}}</li>
+<li>Status: {{.Status}}</li>
+<li>Attempts: {{.Attempts}}</li>
+<li>Next attempt: {{if .NextAttempt}}{{.NextAttempt}}{{else}}—{{end}}</li>
+<li>Created: {{.Created}}</li>
+<li>Sent: {{if .Sent}}{{.Sent}}{{else}}—{{end}}</li>
+<li>Dead-lettered: {{if .DeadLettered}}{{.DeadLettered}}{{else}}—{{end}}</li>
+<li>Last error: {{if .LastError}}{{.LastError}}{{else}}—{{end}}</li>
+</ul>
+{{if .CanResend}}
+<form method="post" action="/_gm/mail/deliveries/{{.DeliveryID}}/resend" class="form" onsubmit="return confirm('Resend this delivery?');"><input type="hidden" name="_csrf" value="{{$csrf}}"><button type="submit">Resend</button></form>
+{{else}}<p class="note">Already sent — resend is not available.</p>{{end}}
+</section>
+<section class="panel">
+<h2>Attempts</h2>
+<table class="list">
+<thead><tr><th>#</th><th>Outcome</th><th>Started</th><th>Finished</th><th>Error</th></tr></thead>
+<tbody>
+{{range .AttemptRows}}<tr><td>{{.AttemptNo}}</td><td>{{.Outcome}}</td><td>{{.Started}}</td><td>{{if .Finished}}{{.Finished}}{{else}}—{{end}}</td><td>{{.Error}}</td></tr>
+{{else}}<tr><td colspan="5"><span class="note">no attempts</span></td></tr>{{end}}
+</tbody>
+</table>
+</section>
+{{end}}
+{{- end}}