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/broadcast.gohtml

@@ -0,0 +1,21 @@
+{{define "content" -}}
+{{$csrf := .CSRFToken}}
+<h1>Broadcast</h1>
+<nav class="subnav"><a href="/_gm/mail">Deliveries</a> · <a href="/_gm/notifications">Notifications</a> · <a href="/_gm/broadcast">Broadcast</a></nav>
+<section class="panel">
+<h2>Admin broadcast</h2>
+<form method="post" action="/_gm/broadcast" class="form">
+<input type="hidden" name="_csrf" value="{{$csrf}}">
+<label>Scope
+<select name="scope"><option value="all_running">all running games</option><option value="selected">selected games</option></select>
+</label>
+<label>Game IDs (comma-separated, for "selected") <input type="text" name="game_ids" placeholder="uuid,uuid"></label>
+<label>Recipients
+<select name="recipients"><option value="active">active members</option><option value="active_and_removed">active and removed</option><option value="all_members">all members</option></select>
+</label>
+<label>Subject <input type="text" name="subject"></label>
+<label>Body <input type="text" name="body" required></label>
+<button type="submit">Send broadcast</button>
+</form>
+</section>
+{{- end}}

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

@@ -0,0 +1,32 @@
+{{define "content" -}}
+<h1>Mail</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>Deliveries</h2>
+<table class="list">
+<thead><tr><th>Delivery</th><th>Template</th><th>Status</th><th>Attempts</th><th>Next attempt</th><th>Created</th></tr></thead>
+<tbody>
+{{range .Deliveries}}
+<tr><td><a href="/_gm/mail/deliveries/{{.DeliveryID}}"><code>{{.DeliveryID}}</code></a></td><td>{{.Template}}</td><td>{{.Status}}</td><td>{{.Attempts}}</td><td>{{if .NextAttempt}}{{.NextAttempt}}{{else}}—{{end}}</td><td>{{.Created}}</td></tr>
+{{else}}<tr><td colspan="6"><span class="note">no deliveries</span></td></tr>{{end}}
+</tbody>
+</table>
+<nav class="pager">
+{{if .HasPrev}}<a href="/_gm/mail?page={{.PrevPage}}&amp;page_size={{.PageSize}}">&laquo; prev</a>{{end}}
+<span>page {{.Page}} · {{.Total}} total</span>
+{{if .HasNext}}<a href="/_gm/mail?page={{.NextPage}}&amp;page_size={{.PageSize}}">next &raquo;</a>{{end}}
+</nav>
+</section>
+<section class="panel">
+<h2>Dead-letters</h2>
+<table class="list">
+<thead><tr><th>Delivery</th><th>Reason</th><th>Archived</th></tr></thead>
+<tbody>
+{{range .DeadLetters}}<tr><td><a href="/_gm/mail/deliveries/{{.DeliveryID}}"><code>{{.DeliveryID}}</code></a></td><td>{{.Reason}}</td><td>{{.Archived}}</td></tr>
+{{else}}<tr><td colspan="3"><span class="note">no dead-letters</span></td></tr>{{end}}
+</tbody>
+</table>
+</section>
+{{end}}
+{{- end}}

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}}

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}}