diplomail (Stage A): add in-game personal mail subsystem

Phase 28 of ui/PLAN.md needs a persistent player-to-player mail
channel; the existing `mail` package is a transactional email
outbox and the `notification` catalog is one-way platform events.
Stage A lands the schema (diplomail_messages / _recipients /
_translations), a single-recipient personal send/read/delete
service path, a `diplomail.message.received` push kind plumbed
through the notification pipeline, and an unread-counts endpoint
that drives the lobby badge. Admin / system mail, lifecycle hooks,
paid-tier broadcast, multi-game broadcast, bulk purge and language
detection / translation cache come in stages B–D.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/cmd/backend/main.go

@@ -25,6 +25,7 @@ import (
 	"galaxy/backend/internal/auth"
 	"galaxy/backend/internal/config"
 	"galaxy/backend/internal/devsandbox"
+	"galaxy/backend/internal/diplomail"
 	"galaxy/backend/internal/dockerclient"
 	"galaxy/backend/internal/engineclient"
 	"galaxy/backend/internal/geo"
@@ -301,6 +302,15 @@ func run(ctx context.Context) (err error) {
 	userNotifyCascade.svc = notifSvc
 	lobbyNotifyPublisher.svc = notifSvc
 	runtimeNotifyPublisher.svc = notifSvc
+
+	diplomailStore := diplomail.NewStore(db)
+	diplomailSvc := diplomail.NewService(diplomail.Deps{
+		Store:        diplomailStore,
+		Memberships:  &diplomailMembershipAdapter{lobby: lobbySvc, users: userSvc},
+		Notification: &diplomailNotificationPublisherAdapter{svc: notifSvc},
+		Config:       cfg.Diplomail,
+		Logger:       logger,
+	})
 	if email := cfg.Notification.AdminEmail; email == "" {
 		logger.Info("notification admin email not configured (BACKEND_NOTIFICATION_ADMIN_EMAIL); admin-channel routes will be skipped")
 	} else {
@@ -328,6 +338,7 @@ func run(ctx context.Context) (err error) {
 	adminNotificationsHandlers := backendserver.NewAdminNotificationsHandlers(notifSvc, logger)
 	adminGeoHandlers := backendserver.NewAdminGeoHandlers(geoSvc, logger)
 	userGamesHandlers := backendserver.NewUserGamesHandlers(runtimeSvc, engineCli, logger)
+	userMailHandlers := backendserver.NewUserMailHandlers(diplomailSvc, logger)
 
 	ready := func() bool {
 		return authCache.Ready() && userCache.Ready() && adminCache.Ready() && lobbyCache.Ready() && runtimeCache.Ready()
@@ -359,6 +370,7 @@ func run(ctx context.Context) (err error) {
 		AdminNotifications:    adminNotificationsHandlers,
 		AdminGeo:              adminGeoHandlers,
 		UserGames:             userGamesHandlers,
+		UserMail:              userMailHandlers,
 	})
 	if err != nil {
 		return fmt.Errorf("build backend router: %w", err)
@@ -579,3 +591,72 @@ func (a *runtimeNotificationPublisherAdapter) PublishRuntimeEvent(ctx context.Co
 	}
 	return a.svc.RuntimeAdapter().PublishRuntimeEvent(ctx, kind, idempotencyKey, payload)
 }
+
+// diplomailMembershipAdapter implements `diplomail.MembershipLookup`
+// by walking the lobby cache for the active (game_id, user_id) row
+// and stitching the snapshot fields together with the immutable
+// `user_name` read through `*user.Service`.
+type diplomailMembershipAdapter struct {
+	lobby *lobby.Service
+	users *user.Service
+}
+
+func (a *diplomailMembershipAdapter) GetActiveMembership(ctx context.Context, gameID, userID uuid.UUID) (diplomail.ActiveMembership, error) {
+	if a == nil || a.lobby == nil || a.users == nil {
+		return diplomail.ActiveMembership{}, diplomail.ErrNotFound
+	}
+	cache := a.lobby.Cache()
+	if cache == nil {
+		return diplomail.ActiveMembership{}, diplomail.ErrNotFound
+	}
+	game, ok := cache.GetGame(gameID)
+	if !ok {
+		return diplomail.ActiveMembership{}, diplomail.ErrNotFound
+	}
+	var found *lobby.Membership
+	for _, m := range cache.MembershipsForGame(gameID) {
+		if m.UserID == userID {
+			mm := m
+			found = &mm
+			break
+		}
+	}
+	if found == nil {
+		return diplomail.ActiveMembership{}, diplomail.ErrNotFound
+	}
+	account, err := a.users.GetAccount(ctx, userID)
+	if err != nil {
+		return diplomail.ActiveMembership{}, err
+	}
+	return diplomail.ActiveMembership{
+		UserID:   userID,
+		GameID:   gameID,
+		GameName: game.GameName,
+		UserName: account.UserName,
+		RaceName: found.RaceName,
+	}, nil
+}
+
+// diplomailNotificationPublisherAdapter implements
+// `diplomail.NotificationPublisher` by translating each
+// DiplomailNotification into a notification.Intent and routing it
+// through `*notification.Service.Submit`. The publisher leaves the
+// `diplomail.message.received` catalog entry to handle channel
+// fan-out (push only in Stage A).
+type diplomailNotificationPublisherAdapter struct {
+	svc *notification.Service
+}
+
+func (a *diplomailNotificationPublisherAdapter) PublishDiplomailEvent(ctx context.Context, ev diplomail.DiplomailNotification) error {
+	if a == nil || a.svc == nil {
+		return nil
+	}
+	intent := notification.Intent{
+		Kind:           ev.Kind,
+		IdempotencyKey: ev.IdempotencyKey,
+		Recipients:     []uuid.UUID{ev.Recipient},
+		Payload:        ev.Payload,
+	}
+	_, err := a.svc.Submit(ctx, intent)
+	return err
+}