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/mail.go

@@ -0,0 +1,86 @@
+package adminconsole
+
+// MailDeliveryRow is one line in the mail deliveries table.
+type MailDeliveryRow struct {
+	DeliveryID  string
+	Template    string
+	Status      string
+	Attempts    int32
+	NextAttempt string
+	Created     string
+}
+
+// MailDeadLetterRow is one line in the mail dead-letters table.
+type MailDeadLetterRow struct {
+	DeliveryID string
+	Reason     string
+	Archived   string
+}
+
+// MailData is the view model for the mail page: a paginated deliveries list
+// plus a snapshot of dead-letters.
+type MailData struct {
+	Deliveries  []MailDeliveryRow
+	DeadLetters []MailDeadLetterRow
+	Page        int
+	PageSize    int
+	Total       int64
+	HasPrev     bool
+	HasNext     bool
+	PrevPage    int
+	NextPage    int
+}
+
+// MailAttemptRow is one delivery attempt on the mail detail page.
+type MailAttemptRow struct {
+	AttemptNo int32
+	Outcome   string
+	Started   string
+	Finished  string
+	Error     string
+}
+
+// MailDeliveryDetail is the view model for a single delivery.
+type MailDeliveryDetail struct {
+	DeliveryID   string
+	Template     string
+	Status       string
+	Attempts     int32
+	NextAttempt  string
+	LastError    string
+	Created      string
+	Sent         string
+	DeadLettered string
+	CanResend    bool
+	AttemptRows  []MailAttemptRow
+}
+
+// NotificationRow is one line in the notifications table.
+type NotificationRow struct {
+	NotificationID string
+	Kind           string
+	UserID         string
+	Created        string
+}
+
+// NotificationDeadLetterRow is one line in the notification dead-letters table.
+type NotificationDeadLetterRow struct {
+	NotificationID string
+	RouteID        string
+	Reason         string
+	Archived       string
+}
+
+// MalformedRow is one line in the malformed-intents table.
+type MalformedRow struct {
+	ID       string
+	Reason   string
+	Received string
+}
+
+// NotificationsData is the view model for the notifications overview page.
+type NotificationsData struct {
+	Notifications []NotificationRow
+	DeadLetters   []NotificationDeadLetterRow
+	Malformed     []MalformedRow
+}