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/server/handlers_admin_console.go

@@ -33,6 +33,9 @@ type AdminConsoleHandlers struct {
 	runtime        RuntimeAdmin
 	engineVersions EngineVersionAdmin
 	operators      OperatorAdmin
+	mail           MailAdmin
+	notifications  NotificationAdmin
+	diplomail      DiplomailAdmin
 	logger         *zap.Logger
 }
 
@@ -51,6 +54,9 @@ type AdminConsoleDeps struct {
 	Runtime        RuntimeAdmin
 	EngineVersions EngineVersionAdmin
 	Operators      OperatorAdmin
+	Mail           MailAdmin
+	Notifications  NotificationAdmin
+	Diplomail      DiplomailAdmin
 	Logger         *zap.Logger
 }
 
@@ -92,6 +98,9 @@ func NewAdminConsoleHandlers(deps AdminConsoleDeps) *AdminConsoleHandlers {
 		runtime:        deps.Runtime,
 		engineVersions: deps.EngineVersions,
 		operators:      deps.Operators,
+		mail:           deps.Mail,
+		notifications:  deps.Notifications,
+		diplomail:      deps.Diplomail,
 		logger:         logger.Named("http.admin.console"),
 	}
 }